I was actually thinking about doing this same thing today with a SQL 2012 cluster but since it's active/passive, I didn't want to license both node by installing RS on both and configuring a scaled out implementation.

I did stumble across this article which may help you with your situation.

http://www.mssqltips.com/sqlservertip/2778/how-to-add-reporting-services-to-an-existing-sql-server-clustered-instance/[/url]