You can get the cap off. I boiled some water and held them with tongs so just the end cap was submerged in the boiling water maybe a couple millimeters down. 45 seconds later and the cap popped off like normal, and I scraped the glue out from inside the carto.
As far as cleaning, you can go your usual route once you get the cap off, or you could boil them to clean. I've heard to use slightly less than boiling water to prevent the filler from expanding too much and destroying the carto, but I haven't tried this yet. Will let you know when its time.
Thanks jg. Last week I boiled some cartos without the caps but threw the caps in for sterilization after taking the pan off the stove. It wasn't boiling but still hot. Some of my caps shrunk. 8-o Or melted smaller. Something. So I was curious if that would happen sticking them in actual boiling water.