We don't use EPGP, just a form of DKP.
However we set up a "few" people as designated raid viable off specs that the 25 mans actually utilize from time to time. As in we run 2 main tanks and occasionally need a 3rd. The 3rd tank is considered a tank as their main however they are a designated off spec for heals when we only need 2 tanks. There is a holy pally that is considered a main spec healer and a designated off spec ret. So when we only need 2 tanks the 3rd tank goes holy and one healer goes ret, so we don't over stack heals. We also have a few people designated as a backup tank if needed and a few for DPS.
I think there is maybe 6 total raiders that have these rights.
Designated off spec > regular off spec, because those designated people actually use their off spec to further our progression and are viable in that role. They do an "off spec interest" bid and do not get charged DKP.
There is something so appealing about backhanding someone across the face with a shield.