Clang still has some alternative calling conventions via __attribute__((X)) for individual functions with a bunch of options[0], though none just extend the set of arguments passed via GPRs (closest seems to be preserve_none with 12 arguments passed by register, but it also unconditionally gets rid of all callee-saved registers; preserve_most is nice for rarely-taken paths, though until clang-17 it was broken on functions which returned things).
[0]: https://clang.llvm.org/docs/AttributeReference.html#calling-...