I am also a midwife, and I do home births. I have not yet had the privilege of attending a birth with a poly family, but one of my colleagues in the area has been with the same family twice. Once one female partner had a child, the next time it was the other partner. They have a male partner with the two mothers.
I make it clear that I accept "diversity" in my practice, but I dont advertise it specifically. But, yes, it would be extremely important for poly families to know they can be "out" with you.
Perhaps making your services known within the poly community would let them know it is available?
Just keep swimming, Just keep swimming... ~ Dori
Willow ~ 46yo bi poly woman, married to Bear (formerly known as TB) for 20 years
Bear-Maybe poly/maybe mono straight man, still feeling it out