Also asked on NSMBHD. Ndymario said I should ask it here. Fingers crossed to get an answer...
The NSMB ASM Patch Template is outdated now, and I don't know for how long NSMBHD will still provide a download of it. The next best thing now is the Code Template, which, combined with NCPatcher, allows for writing C++ code for NSMB. Ndy told me NCPatcher is compatible with every DS game, but he didn't know about the new Code Template.