I agree with Jhereg. I think the bottom line is this. I know I don't have any specific right to "have my way", but I think this is probably the most fair and least shady way to do things:
1. The rules of loot should be standardized, well understood, and inflexible beyond standard RNG, at least in terms of TT value. Clearly loot variety falls outside that, and markup is in player hands.
2. Anything that changes this formula, whether permanent or temporary, should be generally announced, and dates+times+locations should be clearly stated.
If either one of those fails to be true, there is the possibility of significant abuse, and little assurance of fairness. As in many things in our current world, Trust is a valuable currency, which is difficult to mint, but easily lost. So systems that are structured to ensure trust, and jealously guarded, will help to ensure customer trust. Just my opinion.