Short answer: there is no way in the current AvroSink to configure the RpcClient, limiting you to just a single host connection (I'm not sure how well it recovers if that host goes down).
The AvroSink is incredibly simplified from what the RPCClient can do and exposes none of the background functionality. Right now, the only way around that is to create a custom sink based off of the AvroSink source code and instead of setting the RPCClient up the way it currently is, you pass into the RPCClient.getInstance() a set of user supplied properties. To implement this in an unsafe way (not checking any of the user's values) would only take a couple lines of code I believe. It is a work around, but it will enable all of the various RPCClient capabilities such as failover or loadbalancing mode and allow it to connect to multiple hosts.
This is something that (I think) there is a JIRA filed for; but if not, it would be very helpful for this to be implemented into the actual AvroSink (and something that should be linked to that is RPCClient.getInstance accepting a Context object, simply for ease of use).