Since Fender never installed any knee levers on their 400/800/1000/2000 PSG's, just about anything you do will require some form of modification.
I imagine that almost any manufacturer's knee lever will work, IF, it can be connected to the pulling "hook" at the changer; either by cable OR, straight rod.
You might check with Ed Naylor or Bobbe Seymour to get their ideas. If anyone would know, these gentlemen would. Do a search of their names on this forum and send them an email.
Since we are talking about CABLES... it can be very easy. The best set up I saw was a guy who screwed a big long "barn" hinge ti the underside, and then scred on a length of rod or pipe that became the actual lever.
The only trick was to get the cable stuff with the turnaround at the peghead end, and the pully that serves as the double link.
Once you got that the cable just connects to a point on the hinge. -- and they fold up well too!