Not really possible. The only way you'd be able to pull it off is going through every one in your raid's targets, and going from there. Not effective at all, and it only works if at least one person has the mob that you want targeted.
It's not possible; Blizzard has intentionally declined to make this functionality available.
Mods can already assign "mark" targets at will, without even needing user input to do so. If we could target marks, it's a short step to mods being able to select the targets for us, which the UI is designed not to allow us to do.