In the control properties dialog, tab "Key Navigation" there is a checkbox named "Set to focus on toggle" (as of LV 2013, see attached image). Combining this activated checkbox with the toggle set to RETURN exactly works as I intended - just pressing ENTER anytime while typing text into the string control activates the Stop button in the VI. So no hassle with "Key up" any more.
(Mid-Level minion.) My support system ensures that I don't look totally incompetent. Proud to say that I've progressed beyond knowing just enough to be dangerous. I now know enough to know that I have no clue about anything at all. Humble author of the CLAD Nugget.