If anything other than random is used, it'll end up with the same set of DT1-members each month. Theymos want it to be more dynamic, and also give newer users a chance to reach DT1.
What's wrong with a limited form of stability in randomness? How about changing DT1 every three months, or put a sitting limit on (no more than six months inclusions). New Legionaries will be born and the list can change, but new DT members do not half to fear immediate retaliation if they oust a scammer that has attracted up a larger member base...
Edit: Maybe Theymos will add a "reporter" filter in too.... give the DT1 to the people that take the time to maintain the forum.