Is there an addon to add frames that show just the tanks?

There is an addon called Who's tanking - frames that does this, though I believe that it requires you to have oRA3 installed as well. However, it let's you maintain your own tank list, letting you assign/remove tanks as necessary, as well as assign a priority to the tanks (based on their location in the list).

1. Get ora3
2. Get SimpleTankFrames

There you go!

This method depends on your raid leaders actually using this function. Mine do.

This is possible using the default UI. I've included 2 screen shots that should show how to do it. Most raid frame addons also have the ability to display them separately (like vuhdo or healbot. I don't know if grid does.)

Mark main tanks like this (or get your raid leaders to)

Enable this option in your interface options to get them to show in the default raid frames

With regard to the click to target with MD, I'd just use clique or a mouseover macro

#showtooltip
/cast [@mouseover,help,exists][@focus,help,exists][] Misdirection;

If you disable the other groups by clicking on the number, while leaving the tanks thing on, it should only display the tanks

This was also possible pre 4.0.1, but worked differently. Also, I'm not sure which, but one of my addons prevented the tab on the left from showing up.

ora2 can work as well.