In response to the trackball and mapping questions raised by hweller, I'll also throw my enthusiastic support behind having a trackball in the hand unit. I too had been thinking of a thumb-ball replacing all/part of a thumb cluster, but your solution seems like it would be much better, assuming that not too much movement of the palm is required to switch between comfortable use of keys and trackball.
As a long time datahand user, I have also found that the mappings of certain keys like () and [] were inconvenient, and wound up re-mapping several things, though I've had to do it in software (I have the Personal, not the Pro). Doing it with xmodmap, while less than ideal, is fine for most keys; the exception is the keys that auto-send multiple "keystrokes". For example, the key that produces a <double-quote> (") in the default mapping is useless, since it actually sends <Shift>+<single-quote> ('), which is an inconvenience due to the fact that I type dvorak (on a datahand that was intended to be used qwerty, because that was what I could afford at the time). The keys that send more than one keystroke by default are effectively dead-keys for me, since there's no way to re-map them. I've also wound up switching the locations of Alt and Tab, so I could hit Ctrl+Alt with just my left thumb, which lets me access many keyboard shortcuts left-handed (e.g. if my right hand is on a (external) trackball, for example). Having the ability to re-map everything at will in firmware will be awesome, but that should be the easy part in all of this.
