TrackPoint would be nice. I've seen one or two keyboards modded with them around the web, even one using a modern version of the trackpoint (with the stick of an older trackpoint attached to the top of the newer one to make it long enough). You may have to file down housings of nearby switches a bit, and of course, there needs to be a hole in the plate (if present) and PCB for the stick to run through. If only there were a keyboard for sale with all of this done "professionally" for you. :(
For a trackball or touchpad, there are some ways to make it more usable. If the pointing device is below the spacebar, it can be used with the thumb; the main issue with this placement is that the keyboard must then also come with a built-in wrist rest, because the pointing device will get in the way of any third-party one. In the old days, it used to be fairly common to see keyboards with a pointing device in place of the arrow keys, or even with arrows on a trackball housing done like mouse buttons.More
(http://deskthority.net/resources/image/7303)
This placement isn't as good as under the space bar though, because you have to stretch your pinky way out if you want to use it without moving your hand, and of course people tend to like their arrow keys.