So I'm thinking of making an NCR character, but I want the Legion duster. If I'm Liked by both factions, how do I go about getting the Legion duster (I understand that you can wear faction armour to offset your NCR rep, but as far as I can see all faction armour either neutralises both Legion and NCR rep or does not effect either)? Or is my only option to enter the DLC being liked by the Legion and neutral with the NCR, and then to turncoat on the Legion after the DLC? Seems a bit annoying.

The reason why I want the duster is I'm looking to maximise my action points for a Energy Weapons build, a concept that doesn't seem to gel well with a Legion play-through (I'm going to play a woman into energy weapons who also uses loads of drugs). Some people don't like to wear the opposing factions armour (such as the duster) - but I don't really have any problem with it, I always interpret it as the character trying to intimidate the Legion or rubbing their faces in it that he went turncoat.

Yeah I can't seem to come up with another sure fire way to get that duster, I have never tried the faction armor trick, but I have seen a lot of people who have done it. Perhaps this will help you RP wise, after Uly' talks to you about how the NCR is failing, your courier decides that the Legion will serve his needs better? just a thought.

Works like this:

Highest reputation determines the Duster. In the case that several factions are the same, NCR takes priority, then the Strip, then Legion (likely because there's more NCR content). Mixed or negative rep leads to Indy, though it also appears good BoS and Follower rep helps you lean towards Indy.

To get the Legion duster, Legion would need to be the highest and literally unmatched (AKA if you're Liked by NCR and Legion, you'd get NCR).

Best way to do that? Lonesome Road would need to be done early, either before you built significant NCR rep OR simply don't confront House or Benny until you complete Lonesome Road, then do so and it resets your reputations with NCR and Legion. Basically this would force you to do Lonesome Road before getting heavily involved in the main quest or NCR quests.