Haha neat idea, OP wants to make an XM accessory for the 2600. Melody pcbs may give you a run for your money in terms of competition. There isn't much else you can add to the 2600 besides Melody and Atarovox...
A cheat device for Atari would be awesome though. I propse this pseudo Game Genie format:
5-digit codes for 4k games:
Hexidecimal address (up to 4kb of address space) plus a one byte value to replace it with.
7-digit codes for bankswitched games:
Hexidecimal adress, check byte (replace with NN only if the value in ROM matches CC), replace byte.
Fairly straightforward, just plug the cartridge into the Cheat device (like a Game Genie)
Plus most of the notable games for it are found much more easily as standalone Camerica game carts and cheaper to collect in this format. So the Alladin deck enhancer was really just a failed marketing experiment. People still collect for it but there were only six games before it got canned iirc. I think only Big Nose Freak Out is rarer as a NES cart than as an Alladin cart. A couple of others not released as standalone NES carts can be found in the quattro 4-in-1 multicarts. There's three versions, Quattro Adventure, Quattro Arcade, and Quattro Sports.
I've considered a cheat device, but I don't know a thing about hex nor do I have the inteligence to make such a thing.