A little more information would be helpful... is this for a single
make/model, one person, a group.

Details about the physical limits would help as well.

First thoughts are to look at the bluetooth keyboards out there,
specifically the Frogpad since they are disabled aware.  Take a look at
<http://www.frogpad.com/FPandYou/disabled.asp> and contact them for more
info.  In addition, there are a number of additional links which might
be helpful on that page.

If the dpad on the device is not usable, that would indicated that the
need might require modifying a joystick as well.

There might be other options as well, but, the more information the
better to point in the best direction.

Beverly Howard [MS MVP-Mobile Devices]