Heat and very good, ie accurately measured, Allen keys are the trick.
Obviously the ability of the key to deliver the required torque is dependent on it being a very close fit in the screw and the proportional inhibiting of the locking medium will increase the smaller the thread.
Using a propane torch will get the locking medium to melt but in a case like this it is far better to not pussy-foot around! Propane heat won't melt the screw or barrel. It might, if you held the torch on it for hours on a really hot day, weaken the alloy of the lever, but I doubt it. What you do have to do is make sure it's as hot as buggery before you try and turn the key and heat the key in-situ in the screw. Yes, this means you'll have to use a rag to hold the key and don't try holding the lever in a vice. All a vice is in this instance is a very big heat sink! You'll waste a LOT of gas getting it really hot! Using rags to hold stuff will, if you're careful, revent you from burning yourself. Thing is with heat sometimes shit happens and you'll burn yourself. Oddly enough this will always be your own fault so take a lot of care and if you do get a blister? HTFU and get over it. Pain is nature's way of telling you you've done something dumb!