Thanks for letting me know the score, and for the tips on working around the bug.
I've tried the GRANT SELECT and ALTER EXTENSION POSTGIS UPDATE methods you've described, but to no avail. It appears that the rds_superuser role - the most-privileged user role available to me on RDS - still can't reclaim permissions on the geometry_columns table.
At this point, I think a hard upgrade in a brand new database is my main option. Or, perhaps, emptying my current DB and reinstalling PostGIS from scratch.