Hi,
your post is a few months old. but i see that you have no reply for your requirement.
Solution:
After deploying your custm content model successfull, you should implement a custom Action class (see SDK example CustomAction). In the executeImpl(Action action, NodeRef actionedUponNodeRef) call nodeService.setProperty(actionedUponNodeRef, qnme, value), where qname is the quallified name of your custom property. then define a composite rule (in Web explorer) for the folder in question, the rule must be as follows:
-when document comes specialize type(with condition such as based on mimetype)
-when document comes run custom action that you implements previosly.
ihope it helps.
