@acm2010 Thanks for your reply…
It’s not that I specifically want to input the romaji. It’s simply that having to be so much “on my guard” not to trip over this task-difference doesn’t feel central to learning Japanese. I just want WK not to treat this mistake as equivalent to writing “hitotsu” when it should be “hitori” or writing “entrance” when I should write “person.” Those are the mistakes that matter, because they reflect lack of progress in reading and understanding Japanese.
Hm.
Maybe part of why this is frustrating is that I’m an intermediate learner trying to get to the place in WK where I’m actually seeing some challenging content, and it’s (as everyone knows) slow to slog through the SRS process while getting to my prior learning-level. So I’m tempted to go along typing and submitting pretty fast, because it’s … necessarily pretty mindless (or pseudo-hypnotic, as @koichi says) at this point (and I’m a parent, and have a day job, and…). And when when this is the kind of thing that I get “wrong,” It’s hardly unbearable, but it feel gratuitously irritating. I wonder whether there are other intermediate students of Japanese who drift away from WK, during the 3 trial levels, for such reasons.
I could do what @riccyjay does (adding the romaji to the meaning field), and I appreciate hearing these ideas… yet I’m surprised that this kind of thing isn’t a common-enough experience to warrant being either a plugin or an option within WK’s own interface.

