This is a touchy subject. I started out with a hospital-based CNM group, and asked them if I could do appointments with them at the same time as with my CPMs. The CNMs said no, they cannot see me at the same time - I had to pick one or the other. I ended up stalling until I had my ultrasound (19 wks), paid for by insurance at the hospital, and then told the CNMs I had chosen a homebirth just recently (a little white lie).
I think it depends on your state, the practice itself, etc. Do you have a homebirth midwife, or planning on doing it by yourself (UC)? If you have a home mw, I would ask her what she thinks.
Hope this helps a little - it's just a hard decision either way. It might be fine, and they might absolutely refuse. Hard to say. Sometimes it's easier to be forgiven after the fact than to ask for permission and be denied...