We do limit the metadata available for metadata overrides - but once you go to formula any output should be possible.  Think of this formula. 

If ({{PicklistValue}}, Today(), {{CreatedDate}})  

Then the date output is totally appropriate.  

Ok guys, what are we talking about here?  Is there now a way to override field metadata via the builder?

Sweet, thanks Pat!

Nice use case today - I wanted to have a checkbox checked by default on new records. Normally I would have done this with a condition, and created a separate model for NewObject. But, I wanted this to work with new records created inline in a table, so Override Field Metadata, et voila!