"search Keil support knowledgebase for 'function pointer'"

This is essential advice - there is an Application Note and a key Knowledgebase article which you must read before doing function pointers in C51!!
I don't have the precise references to hand, but I've cited them so often on the Keil forum that they shouldn't take long to find! ;-)

Function pointers are fraught on C51 (been there...); you can end up needing a whole bunch of Linker controls just to get them working at all (without trashing their local variables) - doing all this and banking too sounds like a tall order to me!!! :-0