I managed to get this working by editing UserDefinedForm.php...

This "if" statement inside getRequiredFields():

if ($field->Required) {
    $rules[$field->Name] = array_merge(array('required' => true), $field->getValidation());
    $required->addRequiredField($field->Name);
}

became:

if ($field->Required) {
    $rules[$field->Name] = array_merge(array('required' => true), $field->getValidation());
    $required->addRequiredField($field->Name);
} else {
     if ($field->getValidation()) {
         $rules[$field->Name] = $field->getValidation();
     }
}

Not sure if that's really the best way of doing it but it seems to get the job done. Now, to figure out how to make the error messages more relevant...

Sort-of-fixed my error message by adding this function to my custom numeric field.

	/**
	 * Return the error message for this field. Either uses the custom
	 * one (if provided) or the default SilverStripe message
	 *
	 * @return Varchar
	 */
	public function getErrorMessage() {
		$standard = 'Please enter a number';
        
                $errorMessage = ($this->CustomErrorMessage) ? $this->CustomErrorMessage : $standard;
		
		return DBField::create('Varchar', $errorMessage);
	}

I was also having a problem with validation; if a user entered something into a numeric field and then blanked it again, the error message would remain and the form wouldn't submit unless the field was completed. The field wouldn't even attempt to validate with "required" set to "false", so I cooked something up to overcome this, which I'll post for the benefit of anyone Googling this:

inside getValidation():

        $options['number']   = true;
        $options['required'] = "function(element) {return element.val() != ''}";

You can also use "digits" instead of "number" if you want to make it integer-only.

Hey Max, Great work digging into these issues. Is there anyway you could put together a ticket on open.silverstripe.org and list everything you did (try making a diff file if you can). Sounds like this would be a good feature to include in the release.

Thanks, Will- it still needs work (i18n, non-JS validation, etc.) but I will try to put a ticket together at some point. Great module, by the way.