It seems that my son is taking more time from me than expected...
I got a external floppy cable for my ex 464+, and connected to my HxC. Everything works as expected, so the final step to 6128+ worked fine. I haven't made a new SCART cable, the composite output adapter or connected it to the A520... a four month baby can be very demanding.
@Executioner: Yes, I did read your post and tried your program, and it works as expected.
The point is that Burnin' Rubber won't work using a single instruction. If you need to load a small program from disk to make it work, it doesn't really matter if you have the rest of the game in ROM or in disk (because you'll need that disk to load the game). So the space used by the game is wasted.
If the ParaDOS cartridge had the |game RSX command, that would be a different thing because the cartridge could be used as system or game cartridge directly.
Thinking about that, I wonder why the cartridge ROM pages are mapped so strangely... if they were mapped below 31, the cartriges could have been easily used as ROMbox.