I have not heard much about how Rift uses mob lock and such but seems as of late games tend to stay away from most dps i think.
I guess what I am trying to say is any method I can think of can be "cheated" by players.
- First hit, have a fast player running around hitting the MOBS soon as they spawn and drag them to the group's tank
- Most DPS, high DPS classes engage at XX% of MOB life when other players are fighting them
- Last hit, use fast hitting builds or burst damage builds to steal the last hit
- Longest time aggroed, multiple tanks holding aggro on several MOBS with DPS to kill and roamers to pull