I was tired of manually switching or just plain forgetting to switch to discipline while queuing as healer and questing around as shadow on my priest and tired of having same problem with warrior and damager/tank switch. Since there's also was a tweet about similar feeling @WarcraftDevs recently, I've polished up and uploaded a simple addon that I used in-house that adds a button below "dungeon ready" popup that will switch you to other spec if you need it to match your instance role: Instance Spec.
For example popup when I queued on priest looks like this:
It uses WoW's internal localized strings so it should match any language of client automatically. Feel free to try and suggest any additions.