What other consoles is it possible to develop games for using entirely open source software, apart from the UB?
It would seem to be possible for the Megadrive:
https://github.com/Stephane-D/SGDK
Has anyone tried SGDK?
I should imagine its possible to build software for quite a few consoles, especially for the older ones like pre-2005 like the NES, Dreamcast and the PSP.