In writing the chain_maker parametric plugin, I ran across a couple of issues. I wonder if they can be addressed though some sort of work-around.
Issue 1:  I wanted to write the script such that it followed the selected path like in Steve's "spline.txt", but I discovered that the only way I could "activate" the script after the icon was pressed was to click in the editor window.  Clicking (or actually running the script - I cannot tell which) effectively de-selects the previous selected shape and automatically selects the shape created by the script. 
Issue 2:  When a parametric plug-in script is run by clicking the icon, then clicking (or click-dragging) in the editor window, the parametric shape is created at the editor click coordinates and the origin is assigned at the new shape's "centroid".
Question 1:  Is there some way to store what was previously selected?  I've tried numerous things, but outside of having a separate non-parametric non-plugin script write it somewhere (like a material name or some attribute, etc.), then reading it with the actual parametric plugin script, nothing works.
Question 2:  Are the coordinates of the initial click in the editor window (after selecting the script's icon) stored someplace accessible?  I cannot find it in the documentation.  The availability of these coordinates has multiple applications. 
Steve:  I love the scripting abilities of Anim8or!  If there is anything I can do to help with development of the ASL for all of Anim8or, please let me know.  I have been a programmer for a long time in multiple languages, and have written similar scripting languages for other applications.