You may wish to read what the current pure-capability CHERI Linux user ABI specifies for mremap(), because we (primarily Arm, in conjunction with us) have thought about this, and the conclusion is not "the existence of mremap() makes CHERI undeployable". See https://git.morello-project.org/morello/kernel/linux/-/wikis...

Add a a sliding window aliasing mode to the hardware? You'd set a page table bit saying "check capabilities not against my VA, but those VAs over there"