Yeah, it sounds really interesting. I wonder if it will have a test function like Sappy had? SappyMod15 actually had a test function where you could test the music and didn't crash as much as the original Sappy did.
To be honest, I have only done Sappy music inserting for the GBA, here's a tip when inserting music on a GBA ROM... use the SappyMod15 because the original one has bugs in it. I don't exactly have the knowledge to create the midi files for the customized songs yet. And I haven't done music editing for the NDS yet, but you can try to insert SWAV files from a different NDS ROM to the NDS ROM you're editing, which may alter the sound effects for the sound banks, it can be done using a hex editor called CrystalTile2.