>RISC-V-Zxxx implementation can be as incompatible with any other RISC-V implementations (and even specification) as one wants.
It can't. Extensions have a lot of freedom, but still have to follow the core spec. You can't just submit amd64 as a riscv extension for a myriad of reasons, it clearly conflicts with the base ISA.
This is also why there is a trademark. If you or Intel wanted to deliberately create nonsense to be disruptive, you are allowed to create it, but then you can't call it RISC-V without receiving some fan-mail from lawyers.
What you could do is add a custom vendor extension, maybe XIntel64, which adds a CSR that can toggle between RISC-V and x86 mode.
...although you could certainly implement a Jazelle-style "branch to x86" instruction if you really wanted. It's fine as long as you come up in RVI mode.
Unfortunately you would not be able to repurpose the LSB of jalr targets as an "x86 bit" à la Thumb because the standard requires you to ignore that bit, though of course you could enable that behaviour with a custom CSR.