If we are willing to softfork something in that allows Lamport and other ttricks and re-enabling of OP_CAT (and converting all OP_NOP to OP_SUCCESS), then maybe we can use a different P2SH-like encoding? i.e. instead of OP_HASH160 <hash> OP_EQUAL as in P2SH, create a new P2XSH with template OP_HASH256 <hash> OP_EQUAL. If this template is recognized, then the softfork version expects the script top to be equivalent to a 0xC0-version script as in tapleaf, including OP_SUCCESS. Then you can execute any extensions supported inside 0xC0-version tapleaves in the P2XSH script, including OP_CAT if that is enabled inside tapscript version 0xC0. This lets you avoid any quantum-non-resistant paths on the way to the script.
(Or maybe just make it into a tapleaf so include the version to the front of the script. sort of thing)