How a slight change could open GLA for a future DPS job

[Reynhart]

Indeed, you "shouldn't" but not as "the sytem won't allow you" but as "You'd be stupid to do it".
If you queue as a DRK and all you do is Provoke everything in Shield Oath, you're just trolling, and you'll be kicked right away.

After all, all classes can equip Provoke. They just won't use it if they know how to play. Titan and Topaz Carbuncle also have good enmity modifier
Besides, since DRK would be a DPS, the party should have a "real" tank there.

You could also design DRK to only equip 2-handed swords, so that they won't have a shield to protect them, or give them lower vitatily...it's really easy to discourage DRK from tanking just because they could technically use Shield Oath