I'm patiently awaiting a certain persons entry to see if he's cracked it! That's if he submitsMappers are fine. That said the larger ROM hack I don't think will work for 32X. That only works on a Genesis with no add-ons because it's using the Memory region reserved for the Sega CD and 32X.
Oh and regarding the mapper the SGDK mapper supports up to 3GB+. There are some limitations at present though firstly PCM can only go in the first 32MEG (4MB) of ROM as there isn't bank switching support for it. Secondly the SGDK currently has a bit of an issue with compression where if an image crosses a 512KB bank the compression screws up. This is partially why my 12.5 FPS ROMS aren't being submitted here. Covering all that in a video shortly but my main objective is to cover this competition first