I am assuming it would follow along these lines:

retVal = myUIObj.SetControlProperty ("GD_LINES","Column$","UDF_CUSTOM_FIELD$")
retVal = myUIObj.SetControlProperty ("GD_LINES","Backcolour$","RGB: 115 147 179")

I am figuring there is a "Hidden" property that can be updated but can't seem to find it.

You absolutely can't do it based on line data.  (Mainly due to the question of what would happen if different lines are for different product lines?  There is no script event for clicking on a line.).

The reason you don't see direct answers to such questions is because panel scripts are really hard to get working properly, and that goes double for grid display scripts.  Users can drag columns from the upper grid to the lower grid (and vice versa) making it practically impossible to construct reliable column scripts.

Thanks Kevin.   This particular Client only enters 1-line SO's but its a valid point.

Even if you find the correct command, configuring the event scripts to trigger it properly will not be easy / reliable.