If you could translate the UNL idea into STEEM, it's like a witnesses list that any witness would have. So for example, @blocktrades could have an "UNL" with 50 witnesses that he would like to consider for determining consensus.
But you can't translate that idea literally into DPOS withut loosing some trustless aspects of the protocol.
The idea is just to understand that the UNL list represents "reputation" for each server.
On REDPOS the equivalent of an UNL would be to just include the REPUTATION of each witness as part of the witness rank.
So for example: An account less than 2 years old, could be excluded (by protocol) from witness elegibility. Or if not excluded, acount age would be a factor affecting the "voting power".
RE: Reputational Enhanced Delegated Proof-of-Stake (REDPOS)