I was considering the idea of using a character I created, for an original story I never wrote, and making him a member of the PPC.

Then I remembered his magic would probably be a bit too powerful. Not by his world's standards, really, but eh.
But then again, there is the fact he often fails the magic and blows things up/turns people into animals/makes cakes spontaneously appear...

In my experience GMs don't like to have PPC agents around for characters because of their meta nature. It depends on the nature of the RP, though.

Lots of us have agents that were/are RP characters. I tend to think of separate RPs as alternate universes as far as a shared character is concerned, and the different instances of the character don't necessarily have to have anything to do with each other. They can if you want them to, though. Plenty of PPC agents have some level of awareness that they are characters, even if it's something they push to the back of their minds most of the time.

~Neshomeh