Dietary calcium goes to the bones, which act as a calcium bank for the nerves. Ig's bodies will sacrifice their bone's calcium to keep their nervous system healthy. I don't know if MBD igs normally suffer permanent nerve damage to some degree.
Even though his mobility is limited, he needs the same sized habitat as a normal iguana, with one exception. These igs must be discouraged from climbing, so their habitats are terrestrial in design, being relatively short, yet having the same floor space as a normal ig's habitat. 4 ft. deep by 8 feet wide for adults (I suppose a 3 X 7 size would do), and elevated to about six or seven feet up off the floor for the sake of their mental health. They need enough floor space so they can move around as much aas possible,, because jus like healthy igs, they need as much exercise as they can get. When you let them roam on your carpeted floor, they get even more exercise. Many of these igs can't cope with smooth floors.
Covering 15 years here, there have been at least several members who posted pics of their severely deformed yet happy iguanas. The bends go up and down, and side to side. Some of those igs look grotesque. But they fell into the hands of intelligent caring iguana keepers and thrived. Iguanas have the ability to do catch-up growth if they are stunted, although the bone damage never reverses. Their rubbery bones do re-mineralize and become strong again. On this forum I've seen pics that made me shudder, yet the owners strongly loved their igs who seemed happy.
Even though the bones become strong again, the misshapen bones have defective joints and those igs more easily get hurt from small jumps and short falls, and some have a degree of trouble getting around. They are less coordinated than normal iguanas. That makes their activities more limited than healthy igs, but the right owner can easily adjust his expectations. If one strains a ligament because the joint has the wrong shape, it could be in serious pain, and become less active and even stop eating while it heals. That can be mistaken for the depressed look of an ig who is sick. You can't tell the difference, so a vet visit is needed to rule out infection and broken bones.
So, MBD igs have a good shot at a satisfying and relatively long life, if they are restricted from dangerous activities such as climbing, jumping, and falling. It normally goes without saying that they need a perfect environment.