Although you could find some disagreement from Orthodox sources, it seems likely that, were there to be a mass movement from the Old Ritualists to the ROC, the question of Apostolic Succession on teh part of the Belokrinitskaya Hierarchy would likely be resolved by ekonomia
. I think it very unlikely that it would be found to be absolutely wanting and not able to be remedied by such. I think the same is true as regards the hierarchy of the other priested Old Ritualist Churches.
As to Metropolitan Ambrosii, of blessed memory, from whom the Belokrinitskaya Hierarchy derive their claim of succession, see here