Sound like you're doing the common things for trying to release one--assuming that nothing in the lock is broken. As you probably know the idea is that unequal axle traction can prevent the axle plate from disengaging. Applying one brake at a time and turns forward and reverse might equalize the traction and allow the return spring to release the lock if the lock isn't broken. I think that if the return spring is broken, gently pulling up on the lock might release it. The mechanism is a pin is the shaft that rides in v-grooves on the fork.
I guess there could be some experimenting with blocking one or both axles up and manually turning wheels and power versions of the same thing but it doesn't sound too hopeful. About the only thing that comes to mind is that a dragging brake might make equalizing traction tricky. Something like very unequal tire pressures is even more far-fetched.