Except for the fact that even though it's leaking oil, it might not be enough of a leak to lose an appreciable amount of the particulates and sludge.
Presumably loosing that much oil, the oil leaking should be nice and golden, not looking black and used either.
Honestly though, I'm just making an argument, you should be fine just adding oil till you can fix it *especially* since you still plan on changing the filter.
I wish I could find the article, but Road&Track or Car&Driver or one of the other car mags did a test where they drained a Qt of oil every x miles and topped it off with new oil. I think at some point the filter stopped doing anything but the oil stayed at a mediocre level of crap because they were losing particulates every time they drained a Qt.
