I do the exact same thing as Redcow, but with Vuhdo. If you are click casting there is no point in targeting your tank since you can get all the information you need from your raid frames and can heal them by clicking on the frame. I usually have the boss as my target for Wind Shear or Purge or just to see how much health he/she has left.

Even better, at least with vuhdo you could create a frame that shows only your tank and his target. Or a frame that shows your focus, the set up you choose is what works for you.
Then you can bind offensive spells that will only be used when you cast them on hostile frames.
For example, I have right click: judgements, left click: stun, shift + left click:turn undead.

I'll third this. The functionality that the person is asking for is already built into VuhDo. You can mouse-bind and key-bind your spells and hotkeys through VuhDo, then simply mouseover your tank in the VuhDo grid and click the appropriate mouse button or keyboard key. That leaves you free to target the boss for dispels, etc. without having to set up focus target macros and the like.