Before I go about building my own custom attribute store, are there plans on supporting calling SQL Server stored procedures from the SQL attribute store? Or providing a 'SqlStoredProcedureAttributeStore' ?

You can invoke stored procs using the SQL attribute store. For example, consider stored proc GetValue with parameters @input1. Sample rule is:

c1:[Type == "InputClaim"]=> issue(store = "SqlPolicyStore", types = ("OutputClaim"), query = "exec GetValue @input1 = {0}", param = c1.Value);

You can invoke stored procs using the SQL attribute store. For example, consider stored proc GetValue with parameters @input1. Sample rule is:

c1:[Type == "InputClaim"]=> issue(store = "SqlPolicyStore", types = ("OutputClaim"), query = "exec GetValue @input1 = {0}", param = c1.Value);