To be more precise, there are three things that the cartridge appears to mask out with a Retrode: the level art, the level layouts, and the copy of GEMS used for the SFX and most of the in-game music. In their place you get $00s and $FFs repeated every several KB (I forget the exact number), with some garbage bytes thrown in for some reason. (By the way, the game loads level art and layouts into VRAM and reads them from there.)
Eke's post described how the custom hardware works; that knowledge combined with the dumper he used (which he also said in that post) can be used by someone (like myself, soon) with both to dump the ROM. However, Genesis Plus GX is the only emultor to actually have support for everything put in so far.
As far as patching the ROM goes, one thing makes it easier: there are no indirect jumps, so everything disassembles in one go, making it trivial to find all the access points and replace the code. I might do just that after someone (myself or someone else) releases a raw dump.
