How to update pg_opclass as non-postgres superuser
It turns out there's a (somewhat hidden) permission for users, namely whether they have permission to "update system catalogs directly," which is specified by the rolcatupdate column in the pg_authid system table:

https://www.postgresql.org/docs/9.3/static/catalog-pg-authid.html

This other answer talks about how to set it:

https://serverfault.com/a/300421

Anyhow, once I set it to TRUE for my user, the UPDATE was allowed.

That column in pg_authid appears to have gone away as of PostgreSQL 9.5.