Just found
Quick Adder 2: this, plus a bunch of templates, will do approximately what I need. Problem solved, until it's not! Thanks.
EDIT: well not exactly. The basic problem of needing to switch record monitoring on/off depending on what I'm doing elsewhere in the studio remains. However, Quick Adder 2 does speed up the initial creation to the point where fine-tuning the setting (and making a reasonable default) isn't horrible. Still, REAPER has all of this IO information and could intelligently manage record-monitoring wrt MIDI Thru. If motivated to do so...