I think the separate keypad may be the better answer. I bought a Magic Trackpad thinking I would use the Mobee overlay solution to have a keypad for my MacBook Pro. When i got the trackpad, I noticed the pressue to activate it varies with where you push. A virtual button low on the track pad needs little pressure to push; one near the top can take easily twice as much force.
I asked my dear wife, the "Speed Queen of the Ten-Key," to compare the force difference assuming the trackpad had an overlay. She said the difference in force was a dealbreaker for fast 10-key use.