Thanks for the advice on the spout, guys.
Turns out heat was the answer... I tried soaking in PB Blaster for 3 days, to no avail. I tried hot water, too.
I finally broke out the propane torch, and heated evenly around the lid threads. I could hear the metal creaking and popping, so I knew it was both expanding and moving. I was careful not to heat the glass much. I then used a rag to unscrew the lid - it came off pretty readily.
Thanks again guys - luckily I didn't have to break out the acid - that was my next recourse