Although i haven't programmed the box selection yet. I have some ideas how to realize that.
You requier a Collision algorihtm for example sphere to box( or simpler point to box).
You create a box then you loop through all ents and check them against the box
if they are in the box they are selected if not they are unselected.
I believe to use serval c_traces is quite slow and inacurate.

muffel