F8 picks the nearest mob. Tab cycles between the nearest targets. I'm not sure if they yet have a command for switching back and forth between two targets, but you could check your options and see. If there is one, assign a key to it. (My EQ1 bard used that switch targets key more times than I can count.)
I let the particle effects of my mezz tell my group what mob I'm mezzing. [I don't usually announce I'm mezzing] Most experienced players know what it means.