Indeed!
As one of those "academics in the humanities" with whom you recently spoke I of course agree that "posthuman" is the wrong term.
For reasons discussed in person, which you yourself explained in your article from last week, I still don't think, however, that "ultra-" is the right prefix.
"Ultra-" does indeed mean "beyond," as you suggested. The question is whether "ultrahuman" would mean beyond the human, or would mean beyond the NORM of what is thought of as human. If the former it definitely won't do. If the latter it's less problematic. Here are some definitions from The OED:
:
: 1. Signifying 'lying spatially beyond or on the other side of':
: ultra-stellar
: ultra-terrene
: ultra-terrestrial
: ultra-zodiacal
: ultra-red
: [....]
This is the "beyond" meaning that won't work.
:
: 2. a. With adjs., signifying 'going beyond, surpassing, or
transcending the
: limits of' (the specified concept)
Here one of many examples is actually:
:
: ultra-human
:
: 3. a. Signifying an excessive or extreme degree of the quality or
condition
: expressed by the adjective forming the second element of the compound,
as
:
: ultra-clear
: ultra-clerical
: ultra-critical
Personally, I like "neo-humanism," if "humanism" itself is deemed wrong.
Stan.