Ah ha! I see what you mean.
Well, that's certainly something that could be done, although the problem then is that I can see it giving the impression that Moz/Visconti/Sancuary/whoever is saying "These songs weren't good enought to be on the album, or to be b-sides, but please pay another £15 to hear them!" which... well, I can see many people turning their noses up at that offer.
The only time that I can think of a band doing that is Less Than Jake, who released the b-side off the singles they released off their album Anthem along with other tracks recorded at those sessons as an album called B Is For B-sides but that was released entirely seperatly rather than as an add on with the 'mother album'.
I just can't see people wanting to spend money on an album they already have, b-sides they properly already have, and songs that weren't deemed good enough to be either album tracks or b-sides.
What might work though, if there's enough songs, is to release a stand-alone single with b-sides a few months after the release of the final ROTT single. At least then you get the songs released, but aren't charging an arm and a let for them.