AFAIK, all cheats are ram cheats and are called once per frame--that is whenever keyinput happens which is in most games checked once per frame. On-the-fly toggling of cheats could be done pretty trivially with a 32-bit mask or 64-bit mask (limiting active cheats to 32 or 64 depending on how you wanted to make it) and would hence require 4 or 8 bytes of RAM. Actually code wouldn't be too difficult--it should only require a mov r0,r0,lsr#1 and a movcc .... The hard part would be constructing and displaying the menu, the number of codes would be more fixed, and finding the RAM to actually store the active cheats flags. It'd also use up 2x as much CPU time per frame (or more) which makes it more likely to glitch out in games.
Overall, not impossible to do but it doesn't seem worth all the trade-offs IMHO.
Overall, not impossible to do but it doesn't seem worth all the trade-offs IMHO.