If the tool is truly stuck like you described, it's not going to go anywhere until it comes loose, then it'll move easy. So you don't need to worry about ruining the splines inside the lockring. What you do need to worry about is bending something in the removal process or rounding off the hex end of the tool. Two options from where I see it - 1) Grab the tool with the biggest pair of locking pliers you can get your hands on, then pull straight out with both hands as hard as you can. This will be the fastest and simplest, but might end with a scratched tool and a pair of large pliers flying towards your face if it slips. 2) Take the cassette back off the wheel if you can, then pound the tool out of the lockring from the backside with a very large screwdriver, tapered piece of pipe, or something like that.
edit: looked at the pics again and this is going to be tricky. You need something solid that'll support the cassette/lockring, but let the tool pass through. Since the lockring is so small, you might have to find something that'll fit (loosely) around the head of the tool between the hex end and the lockring in that small space. BB wrench, cone wrench, something like that. Then you can put the supporting piece on the vise with the lockring tool dangling below the jaws, so that when you give the tool a whack from the backside the force goes into moving the tool and not the cassette.
There's not going to be a nice way to get this fixed. It'll come out, but try not to bend anything important in the process.
Last edited by stausty; 05-29-09 at 12:54 PM.