But this would be a bit simple to do a trick with it. You only have to modify the flashcart as if it were an "old" DS game and the dsi would probably boot it without problems. I don't think the only problem is about the headers that are able to boot.
I think the system of the dsi has something...
Ok, I have upgraded my wiikey to 1.9s and the audiofix doesn't work as you were saying.
I have tested it with naruto 4 and starfox adventures and it starts making this awful noise ¬¬