Does anyone already have a tool for ArcMap that will accomplish this task?
In ArcMap, I'd like to automatically have a value calculated to a field simply be selecting a feature. Essentially bypassing the need to open the table and manually enter the value or use the field calculator. The user would select a value from a list of options ("A" for example) in a menu and then each time a feature is selected the value "A" would be calculated into the selected record immediately. The script, of course, would be edited to change the targeted feature class, field, and list of values as needed for the specific project.
I had written a tool in Avenue for Arcview 3 back in the day to do this and found it quite useful. I called it Click and Calc.
Have you ever used Attributes Assistant? Its an addin for ArcMap that might be able to solve your use case. It performs calculations on features after an edit, but you can also run manual updates/calculations on selected features.
It does take a little bit of configuration but its well worth the time.