With conditionals styles can already be written so that journal articles for which the pages-field is empty are listed as "in press" ...
By the way, independently of how this issue will be settled, the term "in press" is common in bibliographies, and it should be made possible to call it from within CSL while allowing for localization (to avoid hard-coded strings). That's not already the case, right?
This doesn't seem a very reliable way to deduce status. What about an online and HTML-only article that will never have page numbers? They exist now, even if not terribly common.
It's probably not the case, b/c nobody's bothered to implement this. It'll be easy enough to change.