I can think of two options. One would be to simply subclass UIDocument and create a new tag with a different implementation of encodeEnd.
Another option would be to use setRedirect="false" and nest the p:document in any JSF component that implements ValueHolder. (maybe a custom component that you used for your file save function) That's basically how the pdf/mail integration works.
I would add that if you do either of those, report back if there's anything you think UIDocument could do to make life easier to re-use for other purposes.
Thanks Norman, I will report back when make some progress on it.
Norman I have logged JIRA. It would be a good feature to have in already impressive Seam PDF generation. Whichever way its done, obviously it would allow to specify serverside path/filename.
I have logged JIRA: