[eluser]OverZealous[/eluser]
I think the issue you are having is that you are setting a value to be the same as it is.
If you need to force a change, why not do it in the controller? Also, you can override save() in your model, which gives you a chance to perform one-off things like this.
Code:
class MyModel extends DataMapperExt {
...
save($object = '', $related_field = '') {
// handle the special field here
$parent::save($object, $related_field);
}
...
}