In the case of the PSP, I think this type of thing can more easily be done in software like dslitemodder suggested, so the hardmods aren't really necessary. I remember Electro saying that RemaPSP is also capable of both rapid fire and macros, but I haven't tried this personally.
There was
PSP rapid fire HEX code for the PIC12C508A posted a while back, but it isn't particularly useful without the source to show the pin functions.
So in any case, there are a few different ways you could do this, but I would give the software a try first.