Hmm. I think this would be rather tricky.
Decker is designed to try to accommodate text input on devices that don't have a physical keyboard, so it avoids exposing low-level keyboard events and doesn't (presently) allow scripts to modify the contents of a text field while it has user focus. Consider how the soft keyboard overlay introduced in Decker 1.19 interacts with unlocked field widgets. While it's easy enough to detect that a user has pressed return in a text field (and typed a newline),
on change do if "\n" in me.text # ... end end
That event handler wouldn't be able to clear out the user's line of input, which would make a scripting REPL or an IF parser input awkward.
I'll give this some serious thought and see if I can come up with a good compromise to allow the sort of thing you're interested in making