Hello Paul,
I have prepared a Python Add-In that should do what you need. I attached a .zip of it. Note that:
- To install the Add-In, run "PythonAddIn.esriaddin"
- You can see the raw code under the "Install" folder
- If you make any changes, run the "makeaddin.py" and then re-install with "PythonAddIn.esriaddin"
If you haven't already, you can download the python add-in wizard here. You can use "addin_assistant.exe" to change the Add-In documentation.
Let me know if it works out for you!
Also, I ran into one limitation. If the user has an attribute table open (from one of the related tables), they will have to close and re-open the table to see the changes. More info here.