Quote - If you wanted to make it parallel you could just copy it and move it up or down, right?

Yes, I thought about that, but this is a task I want to be able to repeat quickly, as I'll be doing it a lot.  With perspectives that have VPs way off (most of the time), this makes the process unworkable.  (also, not to nitpick, but to ensure clarity, I'm not so much interested in parallel lines as I am in simply extending an existing line - I know, that's still parallel, but you know what I mean I think).

I tried to transform a shape I'd made with the line tool in Illustrator (transform>scale), but for some reason illustrator destroys the angle of the line in the process.

I tried making a line segment covering just the line as it appears in the photoref (here, a window sill), then turning it into a guide, turning on snap, and making a new, longer line but that doesn't seem to do jack either.

Quote - Grab the paintbrush and make a left click somewhere on the page. Now before you click again hold down the ctrl key, move your mouse cursor to another location and then left click once. You should see a line appear between your first left click and you last one.

No, when I hold down CTRL the cursor transforms into the move layer tool, and when I left click again elsewhere nothing happens.