I am using ADFS 2.0 for quite some time and I understand how things work. I\'ve done dozen of custom RPs, custom STSes as well as using the ADFS as the relying STS.
You could also try changing ADFS from windows integrated authentication to forms based authentication. You will probably still have to monkey with the freshness property but now your users will have to enter their credentials even if they are on the same network as your AD.
This article explains it pretty simply:
http://social.technet.microsoft.com/wiki/contents/articles/1600.aspx