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> you could burn the top 2b to mark instructions as 2/4/6/8 bytes (or whatever) in length.

FWIW, this is exactly what RISC-V does.

How is this so? I thought RISC-V was fixed length, except for 16 bit compressed instructions. And afaik those aren't identified by a singular particular bit.

See section 1.5 ("Base Instruction-Length Encoding") of the RISC-V spec. It's actually a bit more complex than just using 2 bits (I had forgotten those details), but the basic idea is the same in that there is a fixed cascade of bits identifying the instruction length.

There aren't any standard extensions with instructions >32b yet, but the extensibility is there in the base spec.

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