This is a good idea that i have also been looking into. When ICS gets stable enough we will probably make a custom hboot based on blackrose with expanded system and reduced cache and data on sd in some fashion. But for now i want to keep the img's small enough so as many people can flash them as possible.
BTW alpha5 with gapps is live
I prefer using it with a repartitioned NAND instead of sdcard (Like Sense style), is kinda troublesome and my 8gb has alot of stuffs....