They could also be multiplexed (MSR $x is address/command, MSR $y is data). Or require a sequence of operations (write this magic sequence of numbers to MSR $z). Or memory-mapped/IO-mapped (with the mapping enabled/disabled by MSR or PCI registers). Or be locked by the BIOS during the boot sequence.

But IMO, it probably can't be disabled at all. The "disabling" would be to change the program it runs to a program which does nothing. So there wouldn't be a "disable IME" bit; there would be bits to either make its memory visible to the main CPU cores, or to read/write to its memory, and it's possible that these bits are accessible only from the IME side, or from SMM.