I can confirm that when you get an international op-up, your ticket says:

CLASS
T
Business

So this would be a dead giveaway of an opup to a DL lounge dragon, I'd expect

For other lounges I doubt it will work either - as Delta typically provides the contract lounge with a manifest. That certainly seemed to be the case in SYD when I flew out at the ANZ lounge, and while I was on that manifest by virtue of being ElitePlus, I doubt they update that list with gate upgrades.