What does the one taste of the dharmadhatu really mean?

A Dharmadhatu means ultimate reality. A taste of this is a taste of ultimate truth,

• which achieving 1st dip or experience of Nirvana,
• getting magga-phala,
• becoming at least a sotāpanna,
• achieving any stage of sainthood,
• etc.

All phenomena have One taste.
It's the taste of Making false sense.
Limited sense within limited frame of reference.
Frames of reference and phenomena mutually co-generate and co-maintain each other.
The sentient beings consume them.
Eating them perpetuates Samsara.
Therefore continues delusion and suffering.
Bodhisattva experiences Compassion.
He's beyond frames of reference
Which are Samsara.

interested in any philosophical, literary, or religious answer on what "taste" means there

The quote in Tenzin Dorje's answer reminds me of one of the Pali suttas, which says,

Just as the great ocean has one taste, the taste of salt, so also this Dhamma and Discipline has one taste, the taste of liberation.

Bikkhu Bodhi wrote an essay on that, The Taste of Freedom, which says,

Just as in the great ocean there is but one taste — the taste of salt — so in this Doctrine and Discipline (dhammavinaya) there is but one taste — the taste of freedom

With these words the Buddha vouches for the emancipating quality of His doctrine.

Whether one samples water taken from the surface of the ocean, or from its middling region, or from its depths, the taste of the water is in every case the same — the taste of salt. And again, whether one drinks but a thimble-full of ocean water, or a glass-full, or a bucket-full, the same salty taste is present throughout. Analogously with the Buddha's Teaching, a single flavor — the flavor of freedom (vimuttirasa) — pervades the entire Doctrine and Discipline, from its beginning to its end, from its gentle surface to its unfathomable depths. Whether one samples the Dhamma at its more elementary level — in the practice of generosity and moral discipline, in acts of devotion and piety, in conduct governed by reverence, courtesy, and loving-kindness; or at its intermediate level — in the taintless supramundane knowledge and deliverance realized by the liberated saint, in every case the taste is the same — the taste of freedom.

The Pali word for that ("taste of freedom") is vimuttirasa.

rasa is literally the word used for flavour -- in the Sabba Sutta for example, the phrase translated "tongue & flavors" is "jivhā ca rasā ca".

I think the word is also used metaphorically to mean "function", for example (from Understanding sense organs):

In addition, the Visuddhimagga describes the sense organs in terms of the following four factors:

• characteristic or sign (lakkhaṇa)
• function or "taste" (rasa)
• manifestation (paccupaṭṭhāna)
• proximate cause (padaṭṭhāna)

Thus, for instance, it describes the eye as follows:

Herein, the eye's characteristic is sensitivity of primary elements that is ready for the impact of visible data; or its characteristic is sensitivity of primary elements originated by kamma sourcing from desire to see. Its function is to pick up [an object] among visible data. It is manifested as the footing of eye-consciousness. Its proximate cause is primary elements born of kamma sourcing from desire to see.

According to the PTS dictionary it might, as well as "function", also be translated "property".

I believe you may be referring to the passage in the Kuddhaka Nikaya -- Udana 5:5 The Uposattha Sutta: Where the Buddha said of his system of teaching and discipline:

“bhikkhave, mahāsamuddo ekaraso loṇaraso. Evameva kho bhikkhave, ayaṃ dhammavinayo ekaraso vimuttiraso. “

“Just as the ocean has one taste, the taste of salt, even so, this teaching and training has one taste too, the taste of liberation. “

The Wisdom Library ( https://www.wisdomlib.org/definition/rasa) gives one of the Pali definitions for rasa as: taste; juice; flavour. Derived from the Sanskrit concepts of ancient theatre and literature, it refers to the over all emotional tone of a piece. The artistic goal was to invoke a mindset which resulted in an emotional response called a rasa.

In classic vedic literary theory, there were said to be 8 rasa. A Work of literary or artistic value should invoke at least one of the first eight of these primary emotional flavors or impacts on the audience:

śṛṅgāra (Erotic), hāsya (Comic), karuṇa (Pathetic), raudra (Furious), vīra (Heroic), bhayānaka (Terrible), bībhatsa (Odious), adbhuta (Wonder),

That being so (no pun intended), The Buddha was creating a play on words in which he compared his teachings to a great work of art, One with an even greater literary value, or artistic merit -- One which invokes a mindset resulting in full emancipation, complete liberation from suffering.

In his commentary to Maitreya’s Mahayanottaratantra-ratnagotravibhanga, Gyaltsab Je says:

Because just as every drop of water is of one taste with the ocean, so every phenomenon is of one taste within natural emptiness.

Geshe Jamphel Gyaltsen's explains:

The absence of inherent existence of the mind of an ordinary sentient being, an arya or buddha is the same [in nature].

And:

The final reality, the tathagata essence, the emptiness of inherent existence of the mind, is inseparable in the sense that all conventional phenomena are of one taste in emptiness. They are all inseparable in emptiness.

It does not mean that all phenomena are one and the same thing, for this would not be a Buddhist tenet. It means that phenomena are identical in that they lack inherent existence. Their lack of inherent existence can not be a basis for distinguishing them. In the same way, we say that sentient beings are identical in that they are subject to suffering while they want happiness. It is important to add “in this or that respect”, for we do not mean that they are one and the same objects.

A phenomenon and its final nature (its emptiness) are one entity. They are inseparable. Just like you can not separate heat from fire, you can not separate emptiness from its basis.

In a commentary to Maitreya's Distinction, it says:

"All phenomena are of one taste in that they are without arising ultimately."

A second interpretation is that it means that, in the perspective of the wisdom directly realizing emptiness, no conventionality appear. A wisdom directly realizing emptiness does so in a non-dualistic manner. Since conventional phenomena are not experienced by that wisdom, we say that such a wisdom does not taste them.

This second interpretation is clearer in the visualization at the end of the Confession Prayer:

Then reflect on emptiness. In emptiness there is no I, the creator of negative actions. In emptiness there is no creating of negative actions. In emptiness there are no negative actions created. Even though there are infinite phenomena, in emptiness nothing exists at all. There is no this and that, no me and you, nothing. In emptiness everything is one taste. From this emptiness, everything comes into existence. Whatever exists is the manifestation of emptiness.

By looking for "phenomena are of one taste", and "in emptiness, everything has one taste", you might find additional material.