Use a String as an E4X Expression in AS3?

Question

I need to use a string to access nodes and attributes in XML using E4X. It would be ideal to have this scenario (with XML already loaded):

var myXML:XML = e.target.data;

var myStr:String = "appContent.bodyText.(@name == 'My Text')";

myXML.myStr = "New Value for bodyText node where attribute('name') is equal to 'My Text'";

I ultimately need to set new values to an XML document using strings as E4X expressions.

Please help!