Since Google can't be chased up (see bug 606198), I turned on mailman's archiving on the JS-internals list (https://lists.mozilla.org/pipermail/dev-tech-js-engine-internals/). However, it only archives since it was turned on, not since the list was created). justdave said this can be rebuilt from NNTP archives, and to file a bug, so this is the bug. The NNTP group is: mozilla.dev.tech.js-engine.internals, and the mailing list is https://lists.mozilla.org/listinfo/dev-tech-js-engine-internals.
justdave: I don't even know where to start with this.
screwing with mailman stuff tonight anyway, I'll mess with this now.
Done. For future reference, the new script that downloads the newsgroup into an mbox file is at notorious:/root/bin/downloadnewsgroup, takes the name of the newsgroup and the file to write it to as arguments. You then copy it in place of the existing mbox file for the list's archive and run /usr/lib/mailman/bin/arch --wipe dev-tech-js-engine-internals