Interesting problem to say the least. I've had a pile of these old girls act up and don't recall your particular problem yet. It could be that the small 4:40 screw that secures the top lever location through the trigger plate in front of the trigger guard has just gotten loose enough to allow the vertical shaft to rise out of its machined shoulder area. That can make it seem like the lever is frozen. Does the top lever itself look a little proud of the top of the receiver? Whether it appears so or not, I'd remove the small screw in front of the trigger guard entirely and see if it appears that the vertical shaft threaded hole is still centered in the trigger plate hole. If not, loosen the rearmost trigger plate screw (Located underneath the trigger guard) enough to allow you to realign the top lever vertical shaft,(with a nail set or a longer 4:40 threaded rod that allows you to apply and maintain lateral pressure), then re-tighten the rearmost screw. Reinstall the small 4:40 screw and hopefully the top lever will move as usual.
If this doesn't get her back in the game, I've got a couple of other ideas, but they are more of a PITA. Hopefully this works.
Since you can't open her up, assume she's got a hot shell in her till you know otherwise.
Let us know how you make out.