First, I have seen treatment like this (and even the absence of prescription, when the functional requirement was there) be credited. That said, before I worried about whether they strictly required a physician (i.e., MD or DO) to prescribe, I would look into whether the hospital/MTF privileging rules basically said that the NP is "prescribing" or has their orders approved by a physician. What I am saying is that before I fought the legal argument of whether or not it is a strict requirement (which pre-supooses that you have not met that requirement), I would see if it is actually the case that the NP did the prescribing. When I say, "I," I am talking about how I would start looking at a case and I am not suggesting how you or anyone else should/would approach a case. I am not (and cannot on this board) give legal advice. Also, understand, that one of the things that horribly complicates giving a simple explanation is that so much in the system (both PEB and VA, but more so at the VA) depends on how they assemble and weigh evidence. Many times, you have to address issues with reference to that first. That is, while it is ideal to have a physician state, straight out, that bed rest is required, if that does not happen, then I would look to what the MEB (or C&P exam) states. The VA worksheets (which the MEBs must also use) tell them to address this point. I would look to the MEB and if it is not clear that you meet the definition of Incapacitating Episodes, I would ensure that the MEB is rebutted.
I recall having read something about this issue in a case. At the moment, I don't have time to research this point and given that the resolution is likely easier by meeting the requirement, I will only offer the above points. Since this is such an important issue and will turn on very fact specific circumstances, again, to say more would likely require review of the case and in depth advice.