DS debugging emulators/hacker grade emulators are not quite as well developed as some of the NES, SNES and GBA ones but you can get most things done. Certainly people have been hacking the binaries in DS ROMs for years, you may even be able to apply it as a cheat if you are lucky.
The main potential issue would be whether there is a general detection for the seals which would then trouble you at some other point in time or (the far nicer thing) is if it is just/boils down to an if else type arrangement for that specific seal that will not trouble the game at any other point (does the game have any point where you are given a blank canvas to draw whatever magic rune you like?).
I can launch into DS binary hacking if you like, though a copy of ndsdis2, crystaltile2, an assembler and a copy of desmume, iDeaS and no$gba debug are the tools and rough usage for everything has been covered in a few places is what most of that would amount to.