Don’t know if this helps, but…
VBRhinoscript has a method WindowPick which emulates a window selection - Python rhinoscriptsyntax does not yet have this method. However, you can script rs.command with _SelWindow or _SelCrossing and achieve more or less the same thing; many people don’t realize you can actually enter coordinates with _SelWindow or _SelCrossing. The selection is view dependent, of course, so you need to set the view you want beforehand.
In the following test script, I artificially created two 3dPoints to indicate the corners of the selection window, then I extracted the X,Y,Z coordinates and converted them into strings. You can of course simply write the strings directly without going through all that, I just assumed you might have some already existing 3dPoints you want to use.
I do not know how to get this stuff directly with RhinoCommon, unfortunately…
import rhinoscriptsyntax as rs
rs.Command("_SelWindow "+strP1+" "+strP2, False)