According to "proper speech" in Buddhism's eight-fold path, is cursing allowed if it's necessary to get the point across?

One of the Five Precepts (pañca-sila) - the basic training rules that is observed by all practicing Buddhist lay men and women - is to follow the precept of abstaining from wrong speech. Wrong speech involves a great many things apart from uttering falsehoods. It includes insulting speech, malicious speech, even gossip. This in positive terms is the practice of Right Speech. It is said that "If you can't control your mouth, there's no way you can hope to control your mind.” This is why right speech is so important in day-to-day practice.

Cursing if it's necessary to get the point across to someone can be termed as harsh speech (spoken with the intent of hurting another person's feelings). The important point is that you were mindful of this slip. I do not see it as a transgression. It is not a sin as that of a transgression against a god, as in other faiths. This is only a trifling unwholesome act done by one who strives to practice the Dhamma, as opposed to the same act by another. What I said above is in keeping with the scriptures.

“Suppose that a man were to drop a lump of salt into a small amount of water in a cup. What do you think? Would the water in the cup become salty because of the lump of salt, and unfit to drink?”
“Yes, lord ….”
“Now suppose that a man were to drop a lump of salt into the River Ganges. What do you think? Would the water in the River Ganges become salty because of the lump of salt, and unfit to drink?”
“No, lord ….”
“In the same way, there is the case where a trifling evil act done by one individual [the first] takes him to hell; and there is the case where the very same sort of trifling act done by the other individual is experienced in the here & now, and for the most part barely appears for a moment.”
— AN 3:99

See this answer for some references to definitions of "right speech", but also "wrathful Buddha".

Assuming that the situation is as the question was phrased, i.e. "if it's necessary", then the answer might be yes it's allowed. This answer gave a (hypothetical and extreme) example of where even lying could be allowed (well not "allowed", exactly, but preferred as the lesser of two evils).

It might be especially allowed in Mahayana Buddhism, which maybe has more of a tradition of "skillful means" (doing something because it's necessary in practice, not because it's theoretically correct).

The concept of a "wrathful" Buddha (which is again, I think, especially a concept taken from Mahayana/Tibetan Buddhism) reminds me of some parents who said once that their children "would never take the boats out without permission: because they know that we would murder them" (the point of the story being that the boats and tides were dangerous for small children, and though the parents' wrath seems to or even claims to threaten the children, it is actually intended to protect them).

Still the number of times when you think it's necessary, versus the number of times when it's actually necessary might be very different -- i.e. it might be (too) easy to justify misbehaviour ("it's necessary ... I want to ... everyone does it"). This is where it might be useful to have a good role model to emulate -- maybe you know a good parent or teacher who would never insult people or use curse words?

Also FWIW I think that "proper speech" appears in two places: it's the fourth of the precepts, and the third factor of the Noble eightfold way.

I think that the precepts seem to be especially focused on social or inter-personal harmony, whereas the eightfold way is especially focused on your own personal perceptions. So maybe these are two/both reasons for "right speech": to promote a well and harmonious society, and as a means towards avoiding being upset yourself.