I recently came into possession of the SNASM2 and SNSERVER install disks, and I've uploaded them in the hope that someone finds them useful. The SNASM2 disk contains the SNASM68K and SNASMZ80 assemblers, SNMAKE and SNLIB, the SNBUG68K debugger, and various other tools for the Sega MegaCD. The SNSERVER disk contains some form of file server for reading and writing files from the MegaCD over the SNASM interface card, and includes some example 68k source code.
I've managed to successfully install the tools via DOSBox on Windows 7 64bit, but I don't yet have a working MegaCD kit so I'm afraid I couldn't test any further.
SNASM2 and SNSERVER disk