Or if the OS doesn't support W+X allocation at all, then you can have a bunch of tightly packed pregenerated trampolines in the binary.

  clo_code:
  4C8B1501100000  mov r10 [rel clo_code+0x1008]
  FF25F30F0000    jmp [rel clo_code+0x1000]
  0F1F00          nop3
  # one page away...
  struct clo_slot {
    void (*func)(void* _R10,...);
    void* data;
    };