Hi, I'm having the same issue issue in 12.2(55)SE6 IP services, did you ever get it working or find a work around?
Web-Auth-TestSW1(config-if)#int fa3/0/1
Web-Auth-TestSW1(config-if)#no sw
Web-Auth-TestSW1(config-if)# ip address 10.x.x.x 255.255.255.128
Web-Auth-TestSW1(config-if)# ip admission webauth1
% This config is not supported on this platform. Try configuring a new rule