I think I remember figuring out how to do this but that was months ago and now I'm struggling with what should be a pretty basic task and after 45 minutes of searching I couldn't find an answer so I figured I would just ask.
I'd like to just add a line - a horizontal line in this case but basically something like specify a starting x,y coordinates in inches and an ending x,y coordinates and have a line between those two positions.
So for example, If I wanted to draw a line between [0,0.50]and [8.5,0.50] how would I do that?
Possibly you didn't find anything relevant because the most logical name for such an object is already taken by another one ("line" as in "a line of text", a property of texts). It is called a GraphicLine in the scripting references.
With that I could find several examples; this one How to draw cross line? shows how to set the individual end points to any value you want, showing it doesn't necessarily need to be horizontal.