Hi @faye, welcome 
I think there is a basis in the vinaya for the ordination of trans people:
At one time female characteristics appeared on a monk. They told the Buddha.
“Monks, I allow that discipleship, that ordination, those years as a monk, to be transferred to the nuns. The monks’ offenses that are in common with the nuns are to be cleared with the nuns. For the monks’ offenses that are not in common with the nuns, there’s no offense.”
At one time male characteristics appeared on a nun. They told the Buddha.
“Monks, I allow that discipleship, that ordination, those years as a nun, to be transferred to the monks. The nuns’ offenses that are in common with the monks are to be cleared with the monks. For the nuns’ offenses that are not in common with the monks, there’s no offense. ”
(source)
I don’t see why it matters whether one transitions before or after ordaining. Transitioning from one gender to another is literally having the characteristics of the other gender appear; from clothes, to hairstyle, to behavior, to hormones, to surgery, etc.
So to my mind, I can’t think of any more explicit endorsement of trans monastics in any religious text other than in the Theravada vinaya 
IMO, we don’t really know what pandaka (the word sometimes translated as eunuch) really refers to. IIRC, around 500 years later when Buddaghosa comments on what pandaka means, it’s clear we have totally lost the cultural context to understand who pandakas were.
Certainly though, people today tend to project their own modern prejudices into these ancient categories. It’s almost like they go “hmm, who do we discriminate against today? ahh, trans people, surely they must be banned for ordination”.
Before this, it was gay men who were suspected pandakas and may have been denied ordination on that ground.
Moreover, to insist that trans women are eunuchs is incredibly hurtful, derogatory and transphobic. Such an extremely harmful view cannot be Dhamma or Vinaya to my mind.
Edit: See also: