Q:

What RPC can I use to query the balance of an arbitrary address

How do I get the balance of an arbitrary address using Bitcoin RPC calls? From the RPC calls I can see you are required to have the private key or an account to use the RPC.
For illustrations sake, if you could demo querying this address:
1HB5XMLmzFVj8ALj6mfBsbifRoD4miY36v
That would be great. Here is the actual balance for this address. 

A:

This has already been answered here:

Calculating address balance
How to get an address's balance with the bitcoin client?

The pull request for this feature (support for watch-only addresses) can be found at: https://github.com/bitcoin/bitcoin/pull/4045

