In a morganatic marriage, George could still remain a dynast; his CHILDREN, however, would have no succession rights. The catch is whether Maria Vladimirovna could legally give her dynastic consent to the match.

My answer is a flat out NO: she cannot give her consent to an unequal marriage, without the Romanov imperial house laws (the reason for which the other dynasts had to settle for morganatic unions) getting changed, to abolish the requirement of equal births.

And that, in turn, cannot happen unless the Russian dynasty were enthroned, which has not been the case in over 100 years. The Grand Duchess cannot change the laws of a deposed house, anymore than the Duke of Castro (who is trying to name his elder daughter, Princess Maria Carolina, as successor to headship of the royal house of Bourbon-Two Sicilies) or the Prince of Naples (who is trying to ensure that his only son, the Prince of Venice, will be succeeded as head of the royal house of Savoy by his own elder daughter, Princess Vittoria) can.

If Maria Vladimirovna had any compunctions, she would make it clear that she is giving only her personal, not dynastic, consent to the match: George could still retain his title as Grand Duke, as well as succession rights. But the issue born to the union should not have any.