I don't have InCopy at hand so can't test it. Try this:
app.activeDocument.printPreferences.pagePosition = PagePositions.CENTERED;
You can also center it horizontally or vertically:
Nope, doesn't work.
But Ingo, you can export to PDF (File > Export) and then print from Acrobat Reader (or whatever) which will center on the larger page size by default.
At least in CS5.
unfortunately this is no option, because we have a large group of InCopy-users, who needs to print center justified with InCopy directly.
(The normal way to do this would be to have a properly-sized InCopy document with the textframe centered in the middle of the page. Any reason you cannot do that? I also don't understand why your users cannot use Adobe Reader. Yet another choice would be to define a custom printer that does the re-centering. But we woud need to know more about your application [like WHY you need this] and also what OS you are using.)