I had opened a new thread with my request as an actual feature request, but sadly, Peter said it's not possible:
These are a different kind of field, so they don't expand, or have any ability to expand. It's been a long standing issue across macOS that you can get return characters into them and then have no idea that they are there.
The good news is that it looks like he might be able to at least add some visual indicator that there's more than one line of text in the field, which would be good enough for me.
-rob.