It looks like all the dists currently on ftp.debian.org have a main/debian-installer section. And according to a comment in another debmirror bug (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=576576#20), debmirror doesn't complain if a section is missing.
It seems like it would be appropriate to simply remove the check for dist name containing -updates. Patch attached.
include-updates-di.patch
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