jrtonkin once bubbled...
If your BC was to fail at that point, you're looking at more weight (28 lb) than you can probably (depending on your physique) swim up. But you said you've got a 16 lb. weight belt... So if you drop that as well, then you're looking at "only" 12 lb of negative buoyancy... probably within the realm of what you can swim; again, that's a "probably", it's really going to depend on your swimming abilities.
Excellent explination jrtonkin.
One other part to remember is the other side of the coin. You don't want the only method of swimming to the surface to put you into an uncontrolled ascent.
Continuing with the model provided... in the situation, you'll want to be able to control 8 lbs of positive bouyancy as your wetsuit buoyancy returns on your ascent. Now if your nearing the end of your dive, you'll have to add back in the added buoyancy of the tank... lets say another 4 lbs. So that means you'll need to try and control an ascent 12 lbs positive at the end of your dive.
If you play around with a spare weight belt, you can get a decent idea of what you can swim with. If you find you can swim with 20 lbs, but not 28 you would still need to ditch weight to be able to swim to the surface, but not 16 lbs. You'd need to ditch 8 lbs if you have a full tank, 4 lbs if you have a 1/2 tank.
So if you made 8 lbs ditchable and 8 lbs fixed, you can now ditch the 8 lbs to be able to swim to the surface, and have a little less positive buoyancy to fight on the ascent. Divide that 8 lbs into two ditchable 4 lb pockets, and the most you have to fight is 4 lbs positive. Since the average adult can displace about 9 lbs with their lungs, that should be controllable to ascend safely.
I personally don't like the idea of ditching a weight belt for anything other than as an absolute last resort (I came close once, but that's a whole other story, which started me thinking about this stuff for the last few months). A better option, as mentioned, is to have buoyancy redundancy... such as a dual bladder BC, a drysuit, a lift bag, etc...
Disclaimer to make those from the 'other' thread happy, both the vocal one and his silent supporters:
I'm only a novice diver and I don't know what I'm talking about, so ignore me.