I am trying to set "Comment" field as being mandatory for a flow transition. Help?

[Nicoleta Andreiana]

I have a small situation where in a flow transition I would like to make sure the user will add a comment. Problem is, it's possible the comment was added before the status was changed, so if I use the Validator path, they would have to add a new comment, which is overkill. However, if I use the Condition for the field to not be empty, they wouldn't even see the transition until a comment would be added, so that can be confusing. 

Question is: can I create a rule where if the Comment field is not empty, the transition is done with no further validation, but if the field IS empty, to pop-up a text informing the user to add a comment in order to change the status?

[Joe Pitt]

I would use a custom field for that. Put it on the transition screen make it required during the transition. You can also the the field as a search criteria.