Is there a way to have the signature box in the definitions pane, but not in the evaluation pane?

Apart from a sticker on your screen, you mean? Not by default, but probably with a plugin that overloads the editor for the interactions. Not sure how, as of now though.

Though in the past, I’ve somehow entered states of DrRacket where the blue boxes in the definition editor were on, and off for the interactions (but the corner arrow was still there). So it may be possible to trigger something directly (with quickscript, without a plugin).