Menno-

A better GUI for EM is on the wish-list- see here:http://docs.limesurvey.org/tiki-index.php?page=Project+Ideas+for+GSoC+2012#Enhance_Expression_Manager

However, I don't have time for that myself.

One additional thought on the syntax highlighting. Although it may be possible to use CodeMirror for it, by saving first, you get the advantage of full and completely accurate syntax highlighting. However, it might be possible create an ajax call to the syntax highlighting engine so that as you modify the contents of the text entry box, the syntax highlighting appears below it.  I've never tried ajax with PHP, so someone else would need to develop that.

/Tom