Is pg_upgrade supposed to work even across multiple major releases? (I know that the README.rpm-dist is mentioning just one major release step, but it could be bound just to actual pg_upgrade which is packaged..)
Yes you should be able to do a one-step upgrade as long as you update from at least a 8.3 server. There are some restrictions depending on the server version that is going to be upgraded though, you can have a look at the documentation for more details here: http://www.postgresql.org/docs/9.2/static/pgupgrade.html