I had something similar in mind a while back...someone on the DIYCNC list called it a nanomill. But, rather than just building it, I decided to use an off the shelf 4x6" cross slide vise.
Same idea...different approach. I love the ability of yours to be clamped to a surface, and do its work that way. My take on it is slightly different.
In fact, I just got some lapping compound, and will start by lapping the ways when I get a few hours to myself.
-- Chuck Knight