Over at ArmChairDIY, there’s a great post about overarm pin routers that also includes free plans for making your own. The plans are from an older Popular Mechanics tutorial by Rosario Capotosto, and they are extremely detailed and easy to follow.
The homemade overarm pin router is designed to be flexible and somewhat modular, and it looks like the spindle holder can be easily modified to hold a rotary tool or drill.
Check it out over at ArmChairDIY
The above photo is also credited to R. Capotosto.
Edit: Here is a direct link to the plans.