I'm in the GPU business. Here, ISA compatibility does not even exist and nobody cares. As long as you own an entire toolchain - a GLES/OpenCL driver, or, as in this case, an Oberon OS and a compiler - nobody gives a tiniest bit of crap about the underlying ISA.
So, I do not see any reason for the Oberon machine to rely on any "standard" ISA. It can even be compiled into some kind of a NISC, and nobody would notice any difference.
Oh I agree about Oberon and GPU's. My argument is for mainstream market for software in desktops and mobile. The ISA usually does matter.
Now, if we're talking GPU's, the effective ISA would be DirectX, OpenGL, OpenCL, etc. They're the standards that software and tooling target. So, does you product get by with not supporting any of those? Or do you have to comply with the your niche's standard interfaces and ecosystems, too?
So, I do not see any reason for the Oberon machine to rely on any "standard" ISA. It can even be compiled into some kind of a NISC, and nobody would notice any difference.