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Depending on the switch you are using you will have to setup what type of load balancing is done on the etherchannel link. Here is the global command:port-channel load-balance {src-mac | dst-mac | src-dst-mac | src-ip | dst-ip | src-dst-ip | src-port...
Just some minor clearifications.You can do an unecrypted page by pointing to an external webserver using the controller. This also gets rid of the 1MB(not 30k) limitation on the controller.
The WLC will have no idea which vlan to use in this scenario. You can do this through a clean access setup with the WLC.http://cisco.com/en/US/partner/products/ps6128/prod_technical_reference09186a0080540637.html
If you are using LWAPP than this feature is built into the controller. Security>Web Auth>Web Login Page. You can do a little Java Script to make the users accept the terms of use you have on the page.