Same thing. Any object that can be placed in FRED, you can measure the distance to. And if you want to measure the distance to the middle of nowhere, just plop down an otherwise unused waypoint there and measure the distance to that, and noone will know better.
All these measurements result in a circular area though, but are simple and need only one distance check. If you want something else, things get a bit more complex.
Say you want the player to be within a square area, things get a bit more complex. Best way to do squares is by checking the absolute coordinates of the player with get-object-x/y/z and comparing that against the boundaries of the desired area. This only works when the boundaries are perfectly aligned with the 3 axes though.
If you want an area that's not circular, but with boundaries not aligned with the 3 primary axes, you approximate it by placing waypoints very, very, very far away for each side of the area, and then checking distance from those. The sides will be ever so slightly concave or convex though, but if you place the waypoints far enough away it will be negligeble.