Soaking in Kroil may help but I know that putting it in a US cleaner will do it and it's about the only way I know of to clean a sealed can well (though I've never needed to clean a sealed one). I put a pan of Kroil in the water and put the parts in that if I'm worried about the finish. It's not as an aggressive cleaning but it doesn't damage aluminum or weaker finishes. Although Hornady does sell a cleaning solution for use on firearms and it's mixed in water. A whole upper will fit in the Hornady US cleaner I have.
For the price of getting 'em cleaned vs. just buying a US cleaner, I'd get the US cleaner and then have it for other stuff too. It works gangbusters for brass and I use that over any other method unless I have thousands. I'm not sure how a mfg. would open a seized can but I can imagine that this would be one way of doing it.
Firing it a few time until it heats up some can make it easier to take apart. To a point anyway.
A US cleaner is also the easiest way to clean a .22 can since they need it regularly. I have one that'll go 4-500 rounds, maybe more before needing to be cleaned, but I have another that needs it every 200 or it'll be a bitch to take apart.