I have read several times that, after detailed studies by experts in the field icing sugar, is pretty ineffective for the reason given above - much of the mite problem is buried in the brood cells, principally drone cells. For that reason, drone cell capping is often practised during the active season. As for Firegazers comment, it is my understanding that getting a newly collected swarm is an ideal time to treat with Oxalic acid. In solution it doesn't keep very well which is why I prefer the evaporation method; also trickling according to everything that I have read is a one-off whereas evaporation can be done several times until the mite drop is reduced to a tolerable level.I think studies have shown that it IS an effective treatment at times in the colony life-cycle when there isn't much brood-rearing going on; at these times (new swarm, in the Winter, not sure when else) it clobbers 30% or so of the mites.
If you try it at a random time of year, it probably wouldn't be effective.