Is there a way to expose the .@ description from a tag in studio 5000 to an ignition HMI without adding a string to my UDT and copying my descriptions over?
No, not with our driver or any other driver I'm aware of. Somebody recently mentioned that FactoryTalk supports it. Whatever the mechanism is, Rockwell does not document it.
The @ descriptions and other metadata are only available in newer Rockwell software, I have never seen a third party platform that supports them at all.