When it comes to a converter like this though, the host system is just used as a pass-through. The 2600 adapter for Coleco (like the Super Gameboy for SNES) I believe was a mostly complete 2600 console in itself, it only reads from the Coleco controller and passes the video and sound through the Coleco.
Coleco used a Z80 and lots of standard parts (almost exactly like the MSX), while the 2600 was pretty much all custom other than the CPU. They were worlds apart.
I wish 7800s weren't so hard to come by though (at least, without costing so friggin' much). I have about 20 7800 carts, but my system died years ago.