I think this should work without using eval:

PHP Code:
var myLoc:String "mc1.mc2.mc3.mc4";

var 
myMCs:Array = myLoc.split('.');

var 
myMC _root;
for(var 
0myMCs.lengthi++) {
   
myMC myMC[myMCs[i]];
}
trace(myMC); 
I'm sure there's a shortcut to writing that as well.

Just an idea that came to me because I know eval is not supported in AS3.