Is there a way to submit a form on an element's #ajax?
You've not mentioned what your ajax callback is doing internal to it, but if all you want is to submit the form upon a change, you can do something like this in jQuery with no drupal involvement:

$('#edit-submit').hide();
$("#edit-field-FIELDNAME-0-value").live('change', function() {
  $('#edit-submit').click();
});


where #edit-submit is the id= in the form html of the submit input button and #edit-field-FIELDNAME-0-value is the id= of your select that you want the form submited upon change.  Throw this and the appropriate drupal behavior wrapper into a js file and you can drupal_add_js() it from your form hook.

ADDITION:

I suppose $('#form-id').submit(); is better than $('#edit-submit').click(); above where #form-id is the id= of your form.

Also, if you are running D6, you'll need the jQuery Update module installed.