Woudn't it have been easier to ask a compiler vendor to modify their compiler (unless it's for hobby, of course)?
At Raisonance we've never actually done this, but we have thought about that in the past, and it does not seem exceptionally complex (especially for the instructions you mention, that are rarely generated by the compiler anyway)
Another way, could be to use the "Code Compressor custom scripts" (not yet relased for 8051, to be honest): this is a sort of language that you can use to tell Code Compressor (a post-linker optimizer) to analyze the code and do basic actions on it: its primary purpose is too look for groups of instructions can can be reduced/eliminated (peephole optimization), but I think one could use it the other way around to replace an instruction with a group of other instructions doing the same thing.

Just some thoughts.

Regards,

Luca (Raisonance)