If you need to override the Prototype submit behavior in MonoRail's FormHelper.FormTag, you will need to use on_submit instead of onSubmit.  This isn't obvious since every other parameter seems to match up to the JavaScript libraries params.

With that out of the way I can hook my custom validation and display logic for a non-domain object ala Billy McCafferty's post.

Here's an example to override the onsubmit behavior:

$Form.FormTag("%{action='login', on_submit='false'}")