I would absolutely not do this. I don't have any hard-hitting research in front of me, but my experiences with Phenibut and my intuition tell me that this is a very bad idea. I was in the same boat as you once. I had been using Gabapentinoids in an uncontrolled fashion to the point I was fully saturated and tolerant to them. I ran out and was experiencing mild withdrawal, but truly I was more perturbed by the inability to get high and feel the mental release that I craved. I'm a major proponent of rectal administration from an HR philosophy perspective along with the ease of doing so, coupled with a great bioavailability.
I wasn't satisfied by eating WAY WAY TOO MUCH Phenibut powder, so figured I would try administering it rectally. It was a severely painful experience in which I had to immediately evacuate my bowels and administer two enemas to staunch the pain and discomfort. The next day, there was blood in my shit and the discomfort was bad.
I think Phenibut is something that our stomachs can tolerate, but that's only because our stomachs are literally full of acid to begin with. Most other areas of the body are not so hardy. Another example would be Ethyl Alcohol. We can drink Alcohol without problems in reasonable quantities, but that doesn't mean you should inject it. I've read case reports of people destroying the lining of their colon by administering enemas with high proof liquor on a one-off occasion.
Just don't do it. There is nothing remotely worth the risk that can't be had taking the drug orally.