Since they are both mips , a pc program to convert r3000 to r4000a? I think this is called code translation. This would be faster running, as less things would have to be emulated. It would be better than an interpreter running on the psp, anyway. This may be how psx games are ported already, by sony.
but it wont work. r3k code _does_ run on r4k (even if allegrex is no r4k :=P). and ofcourse the psp ports are done by recompiling, not by translating a binary :)