Why not something small and simple? Hell, you can do pleny of pentest on an 8bit system, think of the everyday like TV's and bluetooth and stuff.
I am more than sure coding RAM or any memory arangement on an 8bit or fucking even a 16bit system would be a WHOLE lot eaiser than on x86 shit. We already have years and years of global dev on *nix what could we posably do in an OS that BackTrack hasn't already done eh?
But how many Kali equivelents do you see in mcu's? Could be something here, if we arn't talking about something that has been done a billion times already.