I know where you're coming from as a lot of DOMs work on the principle of giving all the cushy duties to the "Chosen Few". That's because they're Twats (and half the time don't even know the correct procedures themselves) but it doesn't make it correct.
If your Rep can't get an answer I suggest you get the phone number of the Area CWU Office and give them a ring (you'll often find it on a CWU Notice somewhere in the coffee room). If he's feeding you a load of B.S for whatever reason then I'm sure you can guess what I think he is.
Stick to your guns and fight this, IF you really want that duty. We had exactly the same problem and several duties had to advertised properly and the "Chosen Few" were kicked off them. DOMs cannot allocate Duties on the basis of Favouritism!
Rep finally got a reply:
In any event the agreement currently out to ballot does not do away with seniority whatsoever, nor does it replace any aspect of the current resourcing agreements the union has in place, including the opportunity of individuals to apply for a full time duty when these become available.
And yes there are those in the office that are the Chosen Few getting away with blue murder and hijacking all the O/T who think they'll jump the queue for better contracts if this deal gets through.