Actually, you can score for "threatening" sub. attempts. Comes under "fighting area control". Obviously though, if you're being bombed on from inside your guard the rest of the round, you're going to have to try a lot of subs to win or draw the round.
Also, while you need to open guard to get most of the available sweeps or subs in, if you're going to break the guy down in closed guard, what's wrong with any of the closed rubber guard positions? Do the jits guys on TUF not know it? I've barely if ever seen rubber guard played on TUF, at least not lately.