The new Higan v105 core that just hit the online updater uses a simplified Super Game Boy routine that makes it much easier to access this feature than with the old v094-based cores. Instead of using the complex, CLI-based ‘subsystem’ launch commands, it looks for the necessary files in the system/BIOS directory whenever you feed the core a *.gb/c file.
To get it working, you’ll need one or more Super Game Boy ROMs from the No-Intro set and the sgb.boot.rom BIOS. I’m going to use the Super Game Boy (World) (Rev. 2).sfc ROM for the original Super Game Boy and Super Game Boy 2 (Japan).sfc ROM for Super Game Boy 2.
Make 2 subdirectories in your system/BIOS directory, one named SGB1.sfc and the other named SGB2.sfc.
Copy your original Super Game Boy ROM into the SGB1.sfc directory and then rename it to program.rom. Copy your Super Game Boy 2 ROM into the SGB2.sfc directory and then rename it program.rom, as well.
Copy your sgb.boot.rom BIOS into each of your SGB1.sfc and SGB2.sfc directories, and rename them to sgb1.boot.rom and sgb2.boot.rom, respectively. There’s a core option to choose which of the two SGB BIOSes you want to use.
There’s some minor visual glitchy stuff happening in Linux, but it plays fine. Windows is perfect when rewind is enabled.