It occurs to me that we can use 彊 to mean "a completely unknown concept that cannot be named". For example if you ask, "when Cthulhu rises from its slumber, what is thefirst thing it will do? Probably it will 彊.
More mundanely, you could also use it as a marker character, given that nobody except for sneaky little QA hobbitses will ever use it in their input.
So www.catb.org/~esr/jargon/html/M/mu.html
That's not on my keyboard... Can I just stick with "ineffable"?