cseq2mid by kr3nshaw at 10:00 PM EDT on May 30, 2018
My search for the sequences used in the StreetPass minigames led me to this post. The person who uploaded the archive also included cseq2midi by soneek, but the version provided doesn't convert the sequences properly.
Using the source code of soneek's program (also provided in the archive), I rewrote cseq2mid from scratch. My version is capable of converting all of the musical sequences without fail.
The program struggles to convert ambient sequences, due to the usage of arithmetic operations and multiple subroutines. If there's anybody who understands more about this than I do, please look at the source code on GitHub and tell me which commands I haven't implemented properly.
The next step is to work out how to create SF2 or DLS files from the data in the StreetPass BCSAR, as some of the sequences sound a bit odd when played back using GM. I haven't been able to find a tool that does this yet. Does anyone know of such a program?
If you're curious about the bits of the format you have listed as unknown, check out this bcseq 'decompiler' I threw together. I posted this in the 3DS thread like two and a half years ago, but it looks like the original link is down.
Hey kr3n, it's me from the Discord, figured this would be the best place to point you to this: https://drive.google.com/open?id=0B2lMV7NovfEOZWt1QTE1ZEp5d0E This has the .BCSARs from the games added in 2017 and I think they deserve a look see just as much as the other ones.