Hi dex13x,
I found this document helpfull, though it is for Designer 7.0 I don't think the schema handling has changed much, http://partners.adobe.com/public/developer/en/livecycle/lc_designer_XML_schemas.pdf.
By default a RadioButtonList is an exclusive group of integer values so I would define an element of type xs:int, or something based on xs:int with the valid values. If you have a RadioButtonList with text values then you could do the same thing with but based on xs:string.
Somehow, I feel I have misunderstood your question but I hope the document helps.
Bruce