The basics (which sounds like you are doing) is set up a trunkport on your router and using sub-interfaces to set that VLAN to your switches again using trunk ports to the router and access-ports to the endpoints. When you hit another router though, it can't transparenlty pass that traffic. It has to route it. You would probably, again using trunk ports and policy based routing be able to set up the end routers to pass traffic from the guest vlan behind the 1721 to the guest vlan interface on the 2691.
If this sound like what you are looking for and need more clarification, please let me know.
--Gavin Budd