As I said in #1188, this should be implemented in all field implementations (including joomla-cms).
Close #1188: Consistent use of XML values for fields 'readonly' and '…
Thanks a bunch @benjaminpick. I'm going to leave it open for a day or so in case anyone wants to comment and we'll get it merged in. Is there a chance that later on when you get more time you could go through and clean up the field implementations you mention?