My own preference is to just use the rules as-is, with the understanding that a magic shield is made of "better material" than any non-magic weapon... so magic shields are knocked out of the defender's hand rather than shattered. You still "lose" the shield, but only for a few rounds, until you pick it up again.
But what to do about magical plusses? Well, in yesterday's rules, I restricted the use of most shields to defending against attacks that do 1d6 damage or less. Let's say that the magical plus of a shield adds one one or more dice to the maximum damage it can defend against. So, a +1 shield can be sacrificed to block a 2d6 blow from a hill or stone giant; a +2 shield could defend against any giant, or other creature doing 3 dice of damage or less; and the very rare +3 shield can block up to 4 dice of damage from a single blow, which means just about any melée attack in LBB D&D. I'd also add the bonus to resist being shattered by a magic weapon, although the magic weapon adds its bonus to the die roll as well, so this really only helps if the shield has a higher plus than the weapon.