The default database cluster is owned by the local 'postgres' user. You can verify that:
Version Cluster Port Status Owner Data directory Log file
9.1 main 5432 online postgres /var/lib/postgresql/9.1/main /var/log/postgresql/postgresql-9.1-main.log
The UCS system has a templating mechanism that creates and maintains the cluster configuration. Particularly, the corresponding pg_hba.conf file always contains the tuple
# Database administrative login by UNIX sockets
local all postgres ident
which grants full access to the postgres user, using local unix domain sockets. Doing
su - postgres
should be enough to have the full palette of pg_* tools, for instance pg_dump or pg_dumpall.
This holds true for all applications that place their databases into the default cluster. If pg_lsclusters (as root) lists more than one cluster you have to consult the documentation of the corresponding application.
Hope this helps as a starting point.