Fran - love that you love the gadget. Bring your clients on, it's stable. Just email in with the requested domains so we can drop the invites.
Now - onto your problem. We do have it so that "Activity Comments" becomes "Activity Comments (external - may be seen by your contact)" when "Email case notification to contact?" is selected for that case.
It's supposed to warn you - kinda like what you're asking for right now. So it begs the questions:
1. Does it not switch to "Activity Comments (external - may be seen by your contact)" for you?
2. If not, can you possibly provide steps to duplicate? If it does show up, but you are missing it, any suggestions to make it more obvious? Icon of a man jumping up and down? A checkbox requiring '[ ] Yes, I get it, my customer will see what I wrote, now just let me save!". Or maybe, just a yellow warning icon?
Paul