It strikes me that a sticker bearing, say, "Get your ticket freedom at
www.debian.org", placed to cover the part of the poster from the text "XSCAPE" to the text ".com" would turn this into a pretty good GNU/Linux poster. It'd be nice to see GNU/Linux ads on the Bakerloo line, instead of the usual nonsense.
Not that I'm advocating anyone should commit vandalism, of course.