Have you tried showers with her? My daughter didn't really like bath water on her face, but she will rinse in the shower because she has "control" of what happens with the water. There is no fear of going under, or gagging on water. I bring my daughter in the shower when I am in there and we both take a shower.
I finally got her willing to lay back in the tub and "swish" her hair around by holding both her hands and letting her lay her head back. Again, with my daughter I feel like she wants control of when and how the water gets on her.
The other suggestion I have is - let her pour the water on her own head. Talk it up, get excited about it, with my son we do things like, "No way you can pour this WHOLE jug of water over your head.." "YOU DID IT!!" -- just making it into a game and getting him involved.
Good luck, I know it can be challenging.