Yes, the org.eclipse.jst.pagedesigner.pageDesignerExtension has an
elementEditFactory option. You can implement an elementEditFactory for
your taglib uri(s) by extending AbstractElementEditFactory. You can then
return an object that extends AbstractElementEdit.
Your element edit can then return an IDropCustomizer. The IDropCustomizer
provides a call back interface to allow you to open a dialog and return
custom creation data (see Javadocs).
If you want to further control how this creation data gets translated into
the actual tag that is added to the document, your element edit can also
return its own ITagCreator by implementing getTagCreator.