I just realized that the i386 processor has aliased registers similar to what I need for the TMS9900. On that machine, [AH,AL] is aliased onto AX. There might be some insights worth stealing here.
Nope, no generated instructions take advantage of the high byte registers (like AH). This register is acceptable for inline assembly, but will not be used in any code produced by GCC. All actions seem to be done with full-width registers. Poop.