Recently, Apple has made it so with a special entitlement, com.apple.private.custom-x18-abi iirc, you can actually use the x18 register, mainly so ARM64 versions of WINE/CrossOver can run outside of Rosetta, since they'll be partially killing Rosetta in macOS 28.
I believe you can find slightly more info on how to use it in some XNU (macOS kernel) tests, namely this one: https://github.com/apple-oss-distributions/xnu/blob/main/tes...