There is similar question about extending dnd functionality: extending VPE (for MBean property) , where it is also said "Dnd to VPE does not provide an extension point." So you cannot change dnd behavior or the inserted html code from external. Simply because VPE do *NOT* have this functionality.
You can add a feature request to JBoss Jira for JBossTools project. At least you can ask for two solutions (or both):
1) about adding rich:column to generated rich:dataTable source code while inserting it from the Palette.
2) to allow customization of the elemnts in Palette.