I don't know if you guys keep up w/ Horde at all, but Horde has some pretty nifty form-handling tools in CVS. The Horde framework provides a very extensible Horde_Form class -- and classes to handle various types of fields -- which includes information about how those fields need to be validated. This would fit pretty well into the Syntax framework. Interesting to note, Horde also has some XML config file stuff that might be a neat way of getting to XML-based entities.
The Form tools are in Horde CVS HEAD and are in use in a few of the Horde applications.