I wonder if you could run the firmware in emulation on a more recent CPU.

I don't think one can even fund assembler docs for a chip so old, rare, and obscure as first SH-1,2,3 families.

This is a weird comment. One of the key features of the SuperH ISA is that it's more or less backwards compatible. I worked on them in the 00s/10s, but I can't imagine they had an entirely different ISA in the 90s. I also know that commercial SH3 emulators exist because I've used them. Heck, Renesas used to ship one with the toolchain.

I'm surprised to hear this, maybe trying to salvage the old binary might have had some sense, but the client already went for a complete re-engineering, recognising the low availability of this rare chip as a great threat.

Yeah, they definitely got rare in the normal supply chain. I'd be surprised to see one in anything other than a japanese design today.

I don’t know anything about these, but found it interesting that people have ported Linux to these chips as they’ve come off patent:

https://en.m.wikipedia.org/wiki/SuperH#J_Core

That is for much more recent, and bigger cores.

The one I talk about has a kilobyte of ROM, and is an original SuperH from early nineties.