Is there a way to access temporary tables of other sessions in postgres?
The lack of access to temporary tables in other sessions is not a matter of permissions, it's a technical limitation of the design. A PostgreSQL backend can't access temporary tables of another backend because none of the usual housekeeping to allow concurrent access is done for temporary tables.

In 9.2 you will want to use an UNLOGGED table instead; this can be visible from other sessions, but retains most of the performance benefits of a temporary table.