If Met. Kallistos did concelebrate, as opposed to just attend, I think it would be the first one-on-one concelebration of an EP hierarch with the OCA.
Metropolitan Isaiah of Denver has concelebrated the Sunday of Orthodoxy with Bishop Basil of Wichita and the now ever-memorable Archbishop Dmitri of Dallas for the last 3 years.
That is not remarkable, as such events are billed as Pan-Orthodox, and the role of the bishops are pretty minimal: the OCA just blends in with the rest of the episcopate. To have them join at the altar, one-on-one (despite the presence of Abp. Justinian, the optics are still striking, especially as Met. Jonah is the primate of an autocephalous Church Met. Kallistos' primate has not recognized) to concelebrate a DL (I take it was pontifical) at a OCA Church is something that it seems the EP has been avoiding.