Good news, contrary to what the above poster stated, you do NOT have to write your own emma tag. Emma comes with a .jar file that you can use with Ant very easily, in fact the docs explain how! Yeah! Basically, you put the emma.jar and emma-ant.jar files in your <ant_home>/lib directory. Then attempt to use the emma tag in ant.
If you have troubles, try taskdef'ing emma.
If you still have problems, feel free to respond. And, dont' try to write your own ant task, that's just a goofy suggestion. I'm half tempted to think the above poster was just pulling your leg a bit.