... a proprioceptive response from uke...
Do you mean "I'm losing my balance" (uke says) in the kinesthetic sense or something along the lines of a refex of simply a response to being grabbed? "Proprioceptive response" seems a bit of a non-sequitur/oxymoron ... when I feel hot (as in burning) I pull back as a response but my sense of heat is not changed. Proprioception being ones awareness of ones body parts and their positions being distinct from the kinesthetic sense of balance and awareness of a state of motion. I'm confused on how you are using the term.
As for my take on ikkyo (I vote as not the most diffculy technique-in case anyone is counting):
Ukes response to sensing their balance being taken is to attempt to regain balance (or escape) which leaves an even bigger opening or simply further unbalacing. If one attempts to apply ikkyo but basically stops with their arms extended in front of them (at waist level with ukes arm captured) then that is not really ikkyo. Once ukes arm is captured and their balance taken (or given as is proper for senior to allow beginners a chance to learn) the motion continues until the arm is pinned onto the mat!
I see this problem all the time with omote and ura - there is a hitch in the technique where tori just stops (after the turn in ura or with ukes arm held at the waist omote- without much of tori arm extension to boot). Keep the technique moving until stopped by the mat!
Kihon with a really skilled uke might require the 'bump' but try to find an Endo clip in ki no nagare with a 'bump' ... besides the one from the mat.
My 2 cents. Maybe you are right, Kevin, the medium is not quite up to the task.