I hadn't tried this until today, but it turns out to work: I can create equations forwards and back in notebook.love, and trigger either selectively based on what blanks I fill in.
The fine print: to switch directions I have to fill in the right blank, clear the old blank, and then type in something outside the old blank (to indicate I'm not going to type further into the old blank).
Works better if I clear the old query first, but who can remember that?
I've decided to just recompute on every keypress and mouse click. It seemed unnecessary, but now I see that there's some benefit from the inefficiency.