Does anyone know if it's problematic to have a postgresql 9.3 slave setup to do streaming replication from a postgresql 9.2 master?
In general, log shipping between servers running different major PostgreSQL release levels is not possible. It is the policy of the PostgreSQL Global Development Group not to make changes to disk formats during minor release upgrades, so it is likely that running different minor release levels on primary and standby servers will work successfully. However, no formal support for that is offered and you are advised to keep primary and standby servers at the same release level as much as possible.
It is quite plain in the documentation - you may try but don't be surprised if it does not work.