I assume you are military? Are you the spouse or the AD member? Tricare policy is silent on the issue of homebirth. I believe at one point it specifically allowed homebirths, but that requirement has been removed. There is one place in the Tricare policy manual that mentions homebirth in passing that was not removed, but I would have to go back and look to find it. What it does address is the kind of providers they will cover for maternity care services: OBs and CNMs. They will not cover CPMs or lay midwives according to their policy. So, the trick is to find a CNM who attends homebirth and secure her in your maternity referral. If there are no CNMs in your area that do homebirth, then you are out of luck for Tricare coverage.
If there is a CNM that does homebirth and is an in-network provider, you just get your referral changed to her. This may mean that you have to drop to Tricare Standard if you are now on Prime and are required to get care at the MTF.
If there is a CNM who does homebirth but isn't an in-network provider, you will have to talk to your Tricare Service Center folks. If there are no other in-network CNMs within 100 miles of your home of record, then they have to allow you to use the out-of-network CNM. They will probably require you to go through your PCM to get the referral change request, and they will likely have to have a special meeting to discuss it, but it should go through. If there are other CNMs within 100 miles of your home, they don't have to approve the homebirth CNM, even if the other in-network CNMs don't do homebirth. BUT, if you are the spouse (not the ADSM), you can still use your point-of-service option and use the out-of-network homebirth CNM; you'll just have to pay more than you otherwise would.
Good luck! It's annoying but keep calling, keep asking questions, keep digging! And feel free to PM me if you have any questions ... I am not an expert, but I have been jerked around by the red tape and I have a little experience trying to get them to approve a referral.