This is another interesting suggestion on how to help people create Cloze questions.
I think that first the Cloze help page should be improved and perhaps normal users could do a better job than specialists .
We could add an external editor and there are some of them in Module and plug-ins as http://moodle.org/mod/data/view.php?d=13&rid=3959&filter=1
Doing it inside the Cloze moodle form, we will have to do it following the Moodle form standards and include a complete validation code for all question types.
Just think of having the shortanswer, numerical and multiple choice question interface on one form.
As a compromise we add the decode and verify function to at least show you the result of your code so that you can do yourself the corrections in a self learning process which is quite in agreement with moodle philosophy.
Pierre