I also support selective cap. There are a few ways to do it (depending which way would technically easiest to implement):
Method 1: Apply it based on race like Gank83 said and lower the DD most for Elves, then Dekans, then Dhans and a small cap decrease for others. The numbers Gank83 brought out seem to be about right (Templar's hits are roughly 10 times bigger than others and they are not supposed to be damage class in the first place), in addition cap Dekans at around 20-30%.
Method 2: Dig up your old Class Talisman plan (
http://forum.playrohan.com/forum/showthread.php?t=21637) and give every talisman a huge amount of Attack %, for Avengers small Attack % and no Attack for Templar talisman. Example: 900% Attack for everyone, 50-100% Attack and say ~500 Agility for Avengers, 0% Attack for Templar talisman. You can test the exact numbers in event server until every damage class deals comparable damage.
Method 3: Same as Method 2 except add Attack Speed instead of Attack. In that case the numbers may be a bit different.
Method 4: Apply the cap onto weapons: Lower the DD on Clubs and Maces to around 15% if it is higher, to 30% for Zhens and others unchanged.