The other day I added 65816 support to the disassembler in my JMON program. It doesn't currently handle instructions that are of different lengths in 8-bit and 16-bit modes. The code can also now be configured at build time whether to support the 6502, 65C02, additional Rockwell 65C02 instructions, and 65816. The was an extension of my existing code and not based on any part of the DEBUG16 program.
The code now also has a new Options command that allows configuring a few things and will likely support more in the future.