Who can legally perform PRP--does it have to be an MD, or can it be an RN or other practitioner?

Who injects PRP depends on the state you are in. In general most PRP injections can be performed by NP and PA’s in most states. 

Depending on the state, different levels of staff can perform the blood draw and the injection itself for PRP. You can reach out to the state medical board if you are particularly concerned.  This is a medical procedure so it should be done under a doctor's supervision (this term also means different things in different states).

Hello alleycatt66, Thank you for your question. 
This is different depending on the state you are in. Some states are rather lenient and others are very strict. I suggest calling the medical board of the state you reside in and they can give you some more information on that.