You can get the original SNASM kit for the SNES and Mega Drive, and the SNASM2 for the Jaguar, 32X and Saturn. The Saturn version was advertised for $3,000 (£1,885), but you can pick one up here for just $399 (£250)!
Here is a news report about the SNASM2 from September 1995, with lots of words I don't understand:
"Integrated with Mirage is a new version of the Sega Saturn debugger software called SNASM2.1 - also supplied by Cross Products. SNASM2.1 is the latest version of Cross Products' real-time PC-based debugger.
New features include comprehensive search facilities in source, memory, and disassembly windows; symbol completion; support for GNU C++ qualified function names; a SNASM2-to-GNU COFF converter; and a standalone COFF loader utility. Working with Mirage, SNASM2.1 lets programmers debug code on the PC while code is being tested on a production Sega Saturn with real-time CD emulation. The full development environment, including the Mirage CD-ROM Emulator, the CartDev interface, SNASM2.1 software, a PC SCSI card and modified production Sega Saturn, is available now for under $8,000."
The producer of the SNASM2, Cross Products, was based in Leeds. It was then bought out by Sega in August 1994, and then sold to Imagination Technologies in September 2001.
Unfortunately I've never had any success programming on old systems. I did manage to program a Matrix computer simulator for the ZX Spectrum in BASIC, however. It had random letters and everything!