There is no reason, that i see, why the whole barrel set needs to come apart in order to put back in the keel rib.
The forend lugs on 21s cradle the tubes, so a loose lug means disassembly of the barrels. And the breech os soft soldered together (but dovetailed) which makes things a little tricky when heating them up. And 4140 is a little harder to solder than older softer barrel steels. But none of those quirky things about 21 barrels would prevent the keel rib alone from being cleaned up and soldered back in.
