Anyone know if I can use a formula for the CC when doing Send Email action from Workflow?

I think it has some limitations, but I think you can.

guess I’ll find out tomorrow morning when my scheduled wf either works, or breaks horribly

I know I've used field IDs before for email fields in there, but not sure if you can use freemarker

Yes. done it before. The thing I ran into though was that workflows require a different separator if you're sending to multiple. I think in the UI other places, it's separated by commas, but there it needs to be semicolons. Or vice versa. hooray consistency!