Hi David,
Please reply to the list, not to me personally.
If you don’t need the region to move (I assume not, since you’re talking about defining dragging forces and so on), you can do something like this:
variable origin_x equal x[ID]
variable origin_y equal y[ID]
variable origin_z equal z[ID]
region spherical sphere ${origin_x} …
ID should be the integer tag of the atom that you’re interested in.