Aha, I’ve fixed it, I believe!
First of all, it seems that “libstdc++.so.6” was indeed being downloaded, by Panda, specifically, and that NSIS was the component looking for a more recent version.
I downgraded NSIS to an earlier version (2.46-7, taken from the repository for Ubuntu “Precise”, to be specific), and that seems to have worked.
Since the installed version of NSIS had been locked, but no other versions were being offered before including the “Precise” repository, I’m guessing that either a system update or my removing old repositories caused it to fall back on what it had available: the newer version. The lock was left in place, however, leading me to mistakenly conclude that I had an appropriate version.
I’m not entirely happy with this solution–it means using an old version of both NSIS and “libstdc++”, the former from a somewhat-old and community-maintained repository, rather than from a more-recent and officially supported one. Nevertheless, it works.
(I intend to leave myself a note regarding this for next time, since I seem to keep bumping into this issue.)