Just make classes unable to queue for any duties above level 30 unless they're in a full premade. Then throw a huge (and I mean HUGE) sign in the player's face telling them that they need to go back to their class instructor to get their Soul Crystal after the Sylph portion of the MSQ is completed.

That way no one will have to deal with telling people to get their soul crystals at level 50, or in some serious fringe cases, level 80.