PRP is typical injected into the Corpus Cavernosa
, not necessarily into the tunica. The tunica albuginea is less than 1mm thick and is a structural tissue. It would be incredibly difficult to be that precise, even with a skilled injector (as I am) performing the procedure. With PRP for Peyronie's, the injection is done within the plaque, which is an inflamed portion of the tunica (a much larger target, proportionally).
That being said, PRP injections specifically for Girth
are typically done via intracavernosal injection, serving two functions:
1. increasing the cellular component of the sinusoids of the cavernosa
2. intentional damage to the tunica and surrounding support structures, creating opportunity for new structural growth
In other words, what you're asking about is done, the technique is performed slightly differently.
Hope this helps.