The current experimental FileSystem API support shows the filesystem URL for each entry, which I love, but it makes it really hard to copy it out. It's hard to make the field wide enough to see the whole URL, and if you can't see the whole thing, you can't copy it.
I can think of a few reasonable options off the top of my head:
Change the field's display type so that even if you can't see the whole URL, clicking on it selects the whole text.
Make it really easy to expand the field [move over other fields when dragging it larger, expand on double-click, etc.].
Add a context menu item to copy it.
This feature is no longer in WebKit.