Hi,
I can't seem to find any MEL command that does that.
However, there is this MGlobal method in the API that does selection based on a rectangular bounding box:
MGlobal::selectFromScreen(x\_min, y\_min, x\_max, y\_max, MGlobal::kReplaceList);
(refer to the LassoTool plugin example)
I've tried this out and it selects all the objects visible in the *active* viewport (you could use setFocus to make the viewport you want active though). One thing to note is that what is visible in a viewport may not be what is seen by the camera (try adjusting the size of the viewport and do a render and you would understand what I mean). So, I guess there is still work to do and the MGlobal::selectFromScreen function may serve as a starting point.
rgds,
pipit