Special Characters in dynamic text

You are correct about split().join(). String.split() will split the text into an Array of elements, cut up at all instances of the passs ed in character. Array.join() will merge all elements as an array into a String, placing a dilimiter of choice. By doing split().join() it's effectively a find and replace. You could even do this:
myString = myString.split().join().split().join().split().join();
And pass in as many find.replace pairs as you want.

If I follow your set up, you should be able to use split().join() at any point. For instance:

myTextField_txt.htmlText = myArray[index].split('β').join('β');

You say its not working. What isn't working about it? ie is there character not showing up at all? Is it showing up as a box? is it showing up as β ? Make sure you are using a font which has the β symbol, like Arial.

Depending on how much you need to convert text in this way, you might want to consider making a function, like this:

function convertEntities(str){
return str.split('&beta').join('β').split('&someOtherEntity;').join('bla');
}

Then you could just do, using the earlier example:

myTextField_txt.htmlText = convertEntities(myArray[index]);
anotherTextField_txt.htmlText = convertEntities(myArray[index2]);

Are you assigning text via TextField.text or TextField.htmlText? If you aren't using htmlText, use that.

Also, trace the XML data after its loaded, see if the greek beta is getting converted to an entity, like β. It probably is. In that case, I don't think htmlText will properly convert it, and you will have to do it yourself, like this:

myText.htmlText = myLoadedText.split('β').join('β');

I often make a function which cleans up entities(like above) that I'll be using in the project.

HTH