Well, I know that it will not work because using a Power Base Converter (PCB, was released with the sega genesis to allow it to boot in SMS mode), it does not work because of the controller (for some reason, you need to use a Master System controller on the genesis with the PCB to play Wonderboy in monster land). You would have to add some different controller support to gens to get it to work. Wikipedia article on the PCB:
http://en.wikipedia.org/wiki/Power_Base_Converter#Master_System_compatibility
It is not like making a whole new emulator, because all you need to do is for Gens to disable the 68000 processor emulation and boot the game through the Z80, which is already emulated (this is what, from a non-technical standpoint at all, I understand to be true just from a hardware perspective). Gens+ and Kega Fusion (the other main Genesis emulators) already have had SMS support for many years, even though they are primarily genesis emulators. Thats what leads me to believe it wouldn't be as difficult.