The same we do manually with window select, now available for scripting

command name: ecncSS
Command Inputs:
Base point: (x,y,z)
Square half value

i.e:

coo = "1,1,1"
cmd = "ecncWS " + coo + " Enter Enter"
Rhino.Command cmd, True

Let say we have a point in X-Y space and want to select everything under and above that point in Z direction, the ecncWindowSelect will help you incorporate window select into your scripting code

Download ecncWindowSelect (ecncWS) RhinoV4