Variable Promotion Requirement

In my workflow, I want to add an item to an environment's pre-deployment approval checklist that will check a variable value. I can do this by going to the workflow, finding the environment I want, clicking the green plus sign next to "Pre-Deployment Approval Checklist", and clicking "Variable". The dialog that appears contains two fields: "Variable" and "Required value".

What goes in the "Required value" field? Does this have to be a static value to test for (e.g., testing that my variable equals "123"), or can I also put regular expressions in there? Can I test that the variable has a value, regardless of what that value is? In my case, I don't care what the value is, but I want to make sure it has one before deploying to my target environment.

Product: BuildMaster
Version: 4.6.1

As written now, that promotion requirement is a very simple text-based match - anything more complicated would require a custom predicate.