Drilling from the headstock is the norm Paul but there are times when a 'powered' tailstock drill can be useful. One is to contra-rotate the drill (versus the work being revolved in the chuck) which is sometimes used to drill very small holes accurately.
The 'powered' drill can take a number of forms – Geometer used a spindle running through the tailstock barrel powered by an electric drill from the rear for instance. If there is an overhead (belt drive) drive available, then an arrangement with a drill chuck/pulley combination mounted in the tailstock (via a morse taper fitted with bearings) can be used.
For the co-ordinate drilling you are considering, an 'overhead' drill combined with a lever feed at the back of the tail-stock would have the advantage of being able to 'peck' small holes which would be a lot more sensitive than racking the saddle to and fro, which is required when using a drill chuck in the headstock. So the idea does have some merit. However, for most occasional needs, a morse taper drill chuck in the headstock will do the job…

Regards,
IanT