speaking of multiple guis, one of the ideas that i did like in guylhem ideas was having guis on sd or cf, and leaving possibility of fbcon only device, then it could be brought to situation when you can manage the stuff similar to xdm, for example qingy, which is an DirectFB based login manager, which im trying to compile for arm now
for pdaXrom stuff - its simple, there should be only --prefix=/opt/pdaXrom added, and assuming you have a sym link or you mount your SD to /opt, it will run, may be with some light scriptiing for library pathes, for GPE, i should look how it's compiling system is implemented, but im pretty sure that its possible to have all gpe's stuff inside one directory
then it all can be packaged, or t least tarballed with some pre- and post- installation scripts, and distributed