Custom discriminators

Hi,
 
I've seen that you can use xsi:type to define discriminators in XML Schema instance documents.
I was wondering if it is possible to define custom discriminators.
 
Why do I need that?
The XML language was not designed with XSD verification in mind.
So I have type="(date|time)". If type="date" there are addional attributes available.
 
In another situation, there is an element with two discriminator attributes:
type="(|password)" and multiline="(true|false)".
I want to assign four different Schema types based on the combinations of those attributes.
Is that possible?
 
Gregor