Note to self (since I keep forgetting the magic incantation) -- this is how you declare entities in E4X XML documents under Askari (my scripting-optimized version of Rhino):

js> var x = 
  <?xml version="1.0"?>
    <!DOCTYPE x [<!ENTITY nbsp '&#x20;'>]>
  <x>test&nbsp;non-breaking space</x>;

js> print(x);
test non-breaking space

js>