Trying to read mirage at only 75 yards can be a bit tricky when you are elevated off the ground. Yes, they are legal to use, and I used one two years ago when I was shooting with irons. However, I found that mirage was not something that could be easily detected from the bench.
Now if I were to shoot the game from the deck (ground) prone with a tight sling, then mirage would be quite useful indeed. My scores would likely not be any worse, and possibly better.
I do have an idea for next year that will involve a way to use a spotting scope (or the rifle scope) for mirage reading. One is allowed any number of wind indicators, but no specifications are given on what these may be. I'll leave it at that.
At this past event, I found that my one wind indicator was too slow in response for the quite variable wind conditions at 25 meters. Though without it, I'd have had a much lower score. At the 75 yard event, it allowed me to shoot a better score than I've shot before, but still not enough to get to the finals.
Mirage has to be about the fastest responding indicator, but does not do too well for judging tail and head winds which affect a pellet's flight more than one would think.
One more comment about mirage, when your target is at 200,300, and 600 yards for the highpower XTC (across the course) game, you are relatively closer to the ground (even while standing at the 200 yard line) than you are at 75 yards at the bench. I have found that mirage is very difficult to see while standing at 200 yards, but at sitting (which is lower than the bench) it starts to become useable.
I got into airguns in the first place as a way to practice highpower XTC at reasonable ranges when I compared the wind drift of pellets to my 80 grain SMK bullets for highpower rifle. 50 yards with an airgun would give me a similar wind drift (in terms of MOA) as my load for 600 yard XTC. At 50 yards when one is shooting prone, I have found that mirage just starts to become visible enough to use with a spotting scope of 20x or more on a warm day.