Should Haiku ever start working on Raspberry Pis, would it be theoretically possible to accelerate the emulation of this or even have a RISC OS VM within Haiku? Prolly not on RPi4, but RPi3 can still run RISC OS and is also ARMv8.
You can theoretically let the guest OS run code directly on the host CPU. That is called virtualization. Haiku currently can’t virtualize a guest OS, but it’s definitely possible if devs are interested.