So the idea is that you want your script to select the corresponding point on the other side of a perfectly symmetric object?
You could (1) get the selection, (2) for each point in the selection get the position in world space with the xform command, (3) search the CVs on the entire object for ones that are near the mirrored position (see below), and (4) add any matches to the selection. This wouldn’t be a very complicated script, but it’s not as easy as if there were a nice option in the select command.
The trick is that you do not want to test for whether the X position is EQUAL to the negative X for the other CV. Rounding errors will infrequently yield the answer “no.”
So instead of
if ($thisx == $thatx)
where $thisx and $thatx are float variables, you want to do something like this:
float $epsilon = 0.00001 // a small number
if (($thisx < ($thatx + $epsilon) && ($thisx > $thatx - $epsilon))
Just like a delta epsilon proof in calculus class, if you ever took that in high school…