for(var i = 0; i < meepsArray.length; i++){
trace(["meep"+i]._x);
}

Try running this. If it keeps saying undefined, maybe the names of your meep instances aren't "meep0" up to and including "meep"+meepsArray.length. In that case, I think the array stores the names of the meeps, and you should replace ["meep"+i] with ["meep"+meepsarray[i]] (supposing the array contains their serial numbers, not their names).

If the _x values all return numbers, then I don't see why it would not work. You are running your code onenterframe, right? Or in a set interval? Either is good, as long as the code keeps looping.