The following information has been extracted from disassembling a SGB1v2 firmware; it should be verified on other SGB revisions.
The SGB firmware explicitly ignores all commands with ID >= $1E. This leaves undocumented commands $1A to $1D inclusive.
Commands $1A to $1F (inclusive)’s handlers are stubs (only contain an
RTS). This is interesting, since the command-processing function
explicitly ignores commands $1E and $1F.