HMOs generally won't cover planned homebirths. The two largest HMOs in my area specifically state that there is no coverage for homebirths or out-of-network prenatal care providers in their Certificate of Coverage.
PPOs may or may not cover homebirth. It may help not to mention that your birth will be out of hospital, and have your midwife help you submit receipts with appropriate billing codes. The codes are no different for a homebirth vs. a hospital or birth center birth. When interviewing midwives, you can ask if they have experience with helping their clients get reimbursed.
If a PPO won't cover the birth, often they will cover prenatal and postpartum care if receipts with appropriate billing codes are submitted. I suppose it is up to you and your midwife to determine how much the prenatal/postpartum care costs relative to the actual birth (wink wink).
I have an HMO, so we'll be paying $2800 out-of-pocket. I considered switching to a PPO during this pregnancy to get more of our costs covered. Looking at the PPOs available to us, I realized that with the deductible and higher charges for out-of-network care providers, and factoring in our typical yearly health care needs, the costs wouldn't be significantly lower with the PPO. In fact, if I ended up transferring to hospital-based care for medical reasons, we could end up paying more.
If you have the option of switching to an insurer who will cover homebirth, you'll need to do those calculations for yourself to decide if it is worth changing.
Good luck!

Carolyn