Removing all that awful paint would be a great idea LadyA.
These locks look wonderful when stripped and buffed up....... with decent knobs.
There is nothing available that I can think of.......... just animal force.
If you can get all the knobs off, and just be left with the spindle, you can refit with new knobs.
If any knob remains stubborn (with one left on the spindle), you could always fit new spindles as well, and discard the old.
These old rimlocks usually come apart quite easily once removed. Probably just a matter of finding the screws to open the lock up.
Freeing up the slider is easy to do then.
And, yes, you may be able to have a key made. Rimlocks are quite crude. They often have only 10 "differs". That means only 10 possible key configurations possible.
I do hope you're able to split the knobs and so get the spindles off successfully. It would be well worth it. I can then tell you how to strip the lock and restore it.