Personally, I like the treadle pedal design.
re. hand control. I'd say it can be done w/some mods. It would be a matter of actuating the pivot point via a long hand linkage as opposed to the foot rocker. A simple temporary mod would be to just attach a tall rod to the HST pedal and try it out to see if a more permanent mod is in order.
If so I would recommend to go to a conversion shop, like those that modify vans into campers as they also setup hand throttle and brake controllers for vehicles. Around my way a major conversion shop is called Phoenix Conversions, I don't know if they're nationwide or not tho....
I also have a 1966 Wheel Horse L&G tractor that has an OEM hand controlled HST and to tell you the truth, I like the foot control much better as it leaves my right hand free to operate other implements such as FELs etc., but then again I don't have a bum knee so I can understand your desire. Just be aware that any FEL work with a hand controller would be a MAJOR hand swapping experience......
Good luck