Using the code below in your custom template you can "hack" in checkbox rather than the default select dropdown:

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Then you just need to style the Select with visbility: hidden (not sure whether display:none will still work).

The issue here is that client.js references the selected box's "selectedIndex" so you cant just replace the form element.

Incidently this also default to "do notify". If you dont mind the default being off, you can replace the select bit with {notifyselect}

Hope this helps someone...