Wikimedia Bugzilla is closed!

I have seen a number of instances where sysops have made rollbacks that should have been 
commented to specify a reason for the rollback. Usually this button will be used for 
reverting vandalism or testing, but sometimes there are borderline cases, such as 
factual inaccuracies. Adding a comment in the edit summary of the rollback helps users 
who later review the history, or users whose work was rolled back.

I would think any of:
* An edit box next to the rollback button
* A javascript popup on clicking "(w/c)" (with comment) next to rollback button
* An edit box next to the rollback button which appears when "(w/c)" is clicked
would be appropriate