CathInfo.com

If only the Pope and the Bishops would consecrate Russia, the Eastern schism would end, and the Greek, Russian, Syrian separated churches would retract their errors, return to communion with Rome, and become Catholic again. That's what we should pray for. The promised Triumph of the Immaculate Heart and the conversion of Russia to the Catholic Faith along with the end of the Photian schism. Till then, there will be no peace in the world or in the Church. God has shown the Hierarchs what is the way to Heaven's Peace. It is up to them now to choose that path, and our duty is to pray and sacrifice for it. Eastern Catholic is a legitimate option. Lapsing into the Eastern Heterodox schism and heresy is not. That is false ecuмenism. It is they who left the Church and they who have to return. 

Orthodox have fallen into error on mainly two or three points, but they have not retracted it for centuries, even after the Second Council of Lyons and the Ecuмenical Council of Florence were called to correct them; they at first signed the decrees, then later relapsed. Mainly on Filioque and Purgatory; then, on the Immaculate Conception and also in consequence on original sin. Their own Liturgies teach them the Mother of God, Whom they call Theotokos, is absolutely sinless. But they interpret the Liturgy wrongly. They say it refers only to Her Personal Sinlessness. And others say differently. Then their Liturgy teaches them to pray for the departed to be purified from sins. But they don't want to believe in purgatory and historically raised some false objections to it. The Eastern Liturgies are all good. Orthodox have fallen into some errors as a result of their long separation from Rome, the Mother Church. Our Lady has a plan for their return.

The holy office is clear that in danger of death one can receive sacraments from the orthodox. since sacraments are always received in eastern churches within the the liturgy it logically follows that you would have to attend the liturgy to get the sacraments. this is sufficient to show that attendance at orthodox liturgy actually could happen pre VII.
you should be careful who you call schismatic especially as a Traditional Catholic. There are whole groups in church history who were labeled schismatic who said that new definitions being circulated were not traditional. Whole swathes of western Christendom declared Leo the Great a heretic for his definition..

In Eastern Europe and Egypt almost no Eastern Catholic attended Orthodox services in the rest of the eastern world however attendance of Catholic and Orthodox ( mostly Oriental Antiochian) at each others liturgy took place frequently. The Holy Office was dealing with the Greeks primarily and with eastern catholics in western and non patriarchal areas. This answer has no bearing on an easterner in his own territory