Looking for Altair 680 monitor ROM image (or other simple 6800software)

You might try the SWTP monitor, a slight variation on the original Motorola MIKBUG monitor. The SWTP manual is here: http://www.opencores.org/websvn,filedetails?repname=System68&path=%2FSy… The assembler code listing is in the back and appears to have been OCR'd, so it could be mangled into source form with a little effort. These monitors use a bit-bashed serial port on bit lines of a 6820 with an external bit timer read off an input bit on the 6820. Might be an annoyance to try to emulate but alternatively one could simply hook into the base-level character read/write routines. There might be more stuff at Michael Holley's SWTP site but I couldn't access it a few minutes ago: http://www.swtpc.com/ (I could send you my own monitor that I use with the 6800 but it's been awhile since I used it .. also wrote an asm, disasm and simulator a few years ago.)