To answer my own question... I actually found a disassembly of sp-s2 on my hdd that elbarto had done - not sure when, but the file date on my machine is 2013.

I've located the routines that load/save game backup data to/from BRAM but I need to do more reading to work out how the block is allocated to a game when it is first saved. Either way it looks like I can hook in and it won't require lot of code to do so...