Hey, this is pretty cool. After some tweaking I was able to build the compiler. Unfortunately I don't have all the deps for zView (they would need to be recompiled by the new compiler anyway) so I could try mxPlay only:
aout: 249 708 bytes
elf after the tos conversion: 232 908 bytes
Considering the fact mxPlay uses gemlib and cflib only it's quite nice result.
Btw, this is really a nice idea. Most of the work is done by linker scripts and the tos converter, there are minimal changes to binutils and gcc after applying Vincent's patches.