No, you cannot AFAIK, and I find that truly annoying. I used to use the lines to make dotmaps and outlines of future city BFCs, but it's hard to see the tiles at the zoom where you can draw lines, as you said.
The latest development version of BUG (not released in an installer, but you can get it on our website as a nightly SVN snapshot) has a dotmapping tool build in that draws outlines for you. It's quite nice, and we're improving it with more features soon.