In order to define a contextual action for cross-plot layers, you need to put your Python script in your Python directory under a directory called Contextual_Cross_Plot_Cross_Plot_Layer.
Go to Tools->Options->Environment->Python Scripts (or Tools->Options->Python->Scripts in INTViewer 5.2) and click "Refresh from Disk". Your script should be listed automatically.
You will now be able to execute this script from the contextual menu of the cross-plot layer:
This script will add one of every type of trend line to the cross plot layer, each in a different color.
This script is attached to the bottom of this article.
The result after the script executes should look something like the following.