I think the effects of HRT and anti-androgens will essentially be a more intense version of what happens to a man who uses Propecia (which is actually a trade name for Finasteride, which is an anti-androgen, tho in much smaller doses than would be used as part of HRT) and/or Regaine (minoxidil). These treatments will act on any remaining hairs on the head, no matter how small they may be to start with. The follicles will enlarge, allowing thicker, longer hairs. The effect is to make it look as though hair is growing where there wasn't any before, but that's not actually the case. On areas of the scalp that are totally bald and hairless (ie. where the scalp is shiny) those drugs don't have any effect and you have to have transplant surgery.
So if you're seeing an advancing hairline, it's just hairs that were very small and barely visible becoming bigger and easier to see. But who cares, really? You're still getting thicker, girlier hair! 🙂