Got it. That knob was rusted on SOLID! I drilled the set screw out then pried the knob off. Fortunately I did very little damage to the knob in the process. I knocked a little plating off but it's barely noticeable. After the knob was off I used a hammer and punch to move the shaft back and forth in it's hole to break up the rust and get it moving again. The shaft was so stuck that I had to tap it hard enough to mushroom the end a little before things finally freed up. I tapped it up and down in place several times then put the pliers on the shaft and started raising and lowering the feed dogs. There's a linkage blocking me from taking the shaft completely out to clean it thoroughly and I don't want to take things that far apart so this will have to do. All that's left is to drill and tap the knob for a new set screw and re-install it. The new set screw will use an Allen wrench so I won't have to worry about messing up the slot again. Things still move stiffly but given a little time and some more oil they will loosen up again.
They don't build things like they used to. The feed dog knob is solid metal. If it was a modern knob it would have destroyed in the process of getting it off.
It's weird. The underside of the machine is spotless. That's the only actual rust I've found on the entire machine and it was rusted solid.
Rodney