Metta for dead father

It's totally proper and in no way wrong to send metta toward one's ancestors. When developing metta it's even good to use such reminder if having difficulty to send metta to all, near and dear, far and unpleasing, for you would not know where former beloved dwell for now. Good also to remind if difficulties with metta toward non-humans, the chicken you like to eat or animals you dislike.

A being who has not been your mother at one time in the past is not easy to find... A being who has not been your father... your brother... your sister... your son... your daughter at one time in the past is not easy to find. ... mother

To be reminded that "all" around had been near and possible dear relatives before or later, is surely the most effective way to learn having metta and act harmless.

It's totally nonsense to tell people not to send metta toward ancestors, actually the opposite would be practiced.

Here also a generous share by Mahinda Wijesinghe in the gift "Gratitude in the Buddha's Teaching", different form the usual teachings of the Niganthas using "Buddha" this days:

...In Buddhism, traditionally, there are three principal objects of gratitude: In the first place are our parents, next are our teachers, and in the third place our spiritual friends.

First of all, let us take our parents. In the modern era, there have been quite a few reported cases of ill-feeling between parents and their offspring. Of course, if one is on bad terms with one’s parents then something is quite seriously wrong. Perhaps it wouldn’t be an exaggeration to say that if one is on really bad terms with one’s parents then one’s whole emotional life, indirectly at least, is affected, perhaps quite seriously. This is where the practice of mettā bhāvana (meditation on friendliness) helps in re-establishing positive relation with the parents.

One has to learn to develop mettā, though some may have suffered at the hands of their parents, but even so, it is necessary in the interests of their own emotional, psychological and spiritual development to get over whatever feelings of bitterness or resentment that they were harbouring towards their parents.

It suggests that negative attitudes towards parents are still fairly widespread in our society. And the Buddha Himself has quite a lot to say about our positive relation to our parents in the Sigalovada Sutta in the Dīgha Nikāya, the collection of long discourses in Pāḷi. The Buddha is represented there as saying there are five ways in which a son should minister to his mother and father (you notice in Pāḷi it’s always mother and father, never father and mother and that isn’t without significance) as the eastern direction: he should think “Having been supported by them, I will support them, I will perform their duties for them, I will keep up the family tradition, I will be worthy of my heritage, after my parents’ deaths I will distribute gifts on their behalf.” ...

(Note that this gift of Dhamma is not dedicated for trade, exchange, stacks or entertainment but as a means to make merits toward release from this wheel)

I have heard multiple teachers say it is okay to use a deceased person as the "benefactor" or "as the person you like or respect". If the purpose of Metta is to develop jhana and then profound feelings of loving kindness for all beings, then any benefactor living or deceased that helps you achieve this state should be acceptable. I'm not aware of any teachings of the Buddha that would forbid this type of practice.

In addition, do not look at the forbidding of the opposite sex as an absolute prohibition. For example, many practitioners us their children as objects of metta.

While Pa Auk Sayadaws instructions are insightful and generally helpful, many of them are directed towards eastern monks, who usually hold a narrow interpretation of a specific tradition.

According to my teacher, it is important to reconcile and forgive major issues we had had with our (now deceased) parents. There is a tremendous amount of emotional energy locked in grudges and negative memories. He advised that we imagine our parents standing in front of us, talk to them about our past issues, hug them, say I Understand Why You Did What You Did, Thank You and I Love You - and then imagine them ascending out of sight.

Because I have special connection with dream yoga, I reconciled with my deceased father in a dream. This works even better, because in dream you can recreate a more complete image and go deeper with your emotions.