If you have the ability to run GBA games (I believe the homebrew enabled DSi has a GBA emulator worth speaking of, or if you are running an enhanced flash cart, which should also have some measure of cheats, then you can hardpatch cheats into the ROM itself with tools like GBAATM
https://gbatemp.net/threads/gba-auto-trainer-maker-gbaatm.99334/
https://gbatemp.net/threads/gbaatm-rebirth-gba-auto-trainer-maker-a-new-hope.564321/
There are also many premade trainers
http://gba.dellicious.de/gbatrainer/ for the GBA, and you can edit saves too, or grab a save from whatever and then cheat on something else.
Theoretically if the DSi emulator is a piece of DSi code and you can cheat in DSi code then you can find where the emulated GBA's memory is inside the code and target that as well.