Custom filters

Is it possible to add additional filters (like File Type) to the FILTER palette of Bridge CS4?

Especially I'd like to have filters on specific items in XMP metadata.

I've found the CustomSortExtensionHandler.jsx sample in the SDK that uses FilterDescription objects.

It seems that these filter objects can be used only on special nodes handled by a Bridge extension and not on any regular file.

The app.defaultFilterCriteria that is an array of FilterDescription objects is read-only, according to the Bridge CS4 JavaScript Reference PDF.

Does anybody have any hint how to add custom filters or use FilterDescription objects on files?

(Originally posted in the wrong forum - Bridge Windows)