Unfortunately, GKOS keyboard open standard (http://gkos.com) seemingly switched to software implementation, thus producing "yet another" unusual on-screen keyboard for mobile phones. IMHO, the GKOS on-screen version, called ComboKey (http://combokey.com/), has all the disadvantages of your typical on-screen keyboard: occupies the precious screen space, has little tactile feedback, and is only good for typing an occasional SMS or a short e-mail reply.
The desired functionality, however, is to be able to write reasonably long and complex texts, such as technical notes from meetings.
A number of fundamental design choices must be made:
(1) Should the keyboard be integrated with the phone/tablet or a separate item? I have seen a number of implementations, such as this
The major problem is that Android relies heavily on finger gestures, and the mouse is not exceptionally useful; yet another is that both hands are occupied. Finally, one-handed chorders are good for typing, but I personally find them difficult for navigation (of course, your thumb may do arrows, but...) One-handed chorders could be excellent with conjunction of wearable computers, but for a phone/tablet? The current choice is to integrate.
(2) Should the keyboard use its own battery or the phone's? Currently I am undecided. On one hand, Arduino uses 20-25 mA of power -- the phone screen consumes way more. On the other hand, since the phone has an extra compartment on the back, why not to add a battery? Hey, the keyboard may even charge the phone! Something to be evaluated in the prototypes.
(3) Closely linked with the above, should the keyboard use Bluetooth or wired connection? If the former, a separate keyboard battery is a must, and both devices will drain the juice slightly faster. Let it be wired for now.
(4) What switches to use? The plan is to source several different types and evaluate.