Yep, this is possible. The superagent will pull cache content from the repositories it knows about: so if you don't want it to pull from a particular repository, simply assign it a policy where that repo is disabled. Taking that to its conclusion, if you only want it to pull from the master, then disable all repos apart from the master in the policy.
Thanks for the response Joe. That's what I would expect to happen, but unfortunately I don't have a test environment with multiple SuperAgents so I couldn't test for myself. Unfortunately when I raised a case to Gold support the analyst gave me a one line response that it wasn't possible.
Update 01/05/2015 - I have to give the support analyst credit as they contacted me over night realising they had provided incorrect information and have since also confirmed Joe's comments.