What are your plans for interfacing that with a computer?
Are you designing custom electronics too to work with the rotarty encoder, and led's inside (I assume there's a power led, etc.)
My only suggestion would be to make a more basic 3 axis version as well, without A axis features or the f keys, probe and 2 aux buttons. The jog dial is excellent. I'd likely buy one.
EDIT: what about qtouch IC's? http://www.qprox.com/ I've used them and they're pretty cool. A qtouch rotary dial as well (like the ipod clickwheel) would be amazing. I don't know of a cnc pendant like that.
Edit2: here's a link specifically to the rotary encoder http://www.qprox.com/products/qwheel_qt510.php