Pretty simple at the basic level:
Create the file $HOME/.imwheelrc containing the following:
None, Thumb1, F11
None, Thumb2, Escape
When done, run "pkill imwheel"
However, the above also kills the "back" and "forward" mappings for thumb_left and thumb_right in Firefox. Which may actually be a bonus for me, since I don't use those, and just get frustrated when I hit them accidentally!
So imwheel DOES work, albeit maybe not the latest/greatest way to accomplish the task. But it does work for what I want. I'd still like to know if there is a "better" way however.