After looking at the referenced report about this bug, I'm not at all convinced that this 'bug' even needs a fix - there is a clear workaround documented in the thread linked from the bug report and people claiming that the workaround works. Do you have evidence that the workaround does not work?
The workaround is per site solution. So one must add a line in urls.local for every ad site. The fix in the official svn (the one I've attached to the bugreport) fix it completely.
I can't see any reason why you spent any time devising an NMU for such a bug.
This bug is very annoying. On some/most pages it pops up more than one download window. If someone planing on using bfilter on lenny, than he/she must add all ad urls to the urls.local or recompile it from source. The point of bfilter contrary to adblock is that it don't need any blacklist to work. It's a so little fix and affect the usability of bfilter so much.