Hey all -- I am the main engineer over at askmrrobot.com.
You all are correct that we don't put a weight on the resistance effect of the Mirror, thus you have to manually lock that trinket in as BiS, and basically ignore the ranking of it. It is a unique proc that would be quite difficult to rank as the other trinkets are ranked.
A few updates ago, we made significant improvements to the way that we rank and model tanking trinkets. Firstly, we built a proc simulator to get accurate average up-times for trinket procs and use effects. Then, we (finally!) implemented code to properly apply diminishing returns to tanking procs, which give large chunks of avoidance all at once.
We are in the process of updating everything for 4.2. One change that we will be implementing at this time is some extra logic around the avoidance cap, which apparently will be much easier to reach... if there is no scaling introduced as blizzard hinted at before cataclysm. Up until now we have sort of ignored it because you can just barely reach it in current gear. We can't get away with being lazy anymore though

If anyone has any feedback or a tanking "wish list" for changes/improvements to askmrrobot.com for 4.2, now is the time to tell us!