The specific question of the Eastern Churches legitimate patrimony was taken up by the Second Vatican Council, in its decree Orientalium Ecclesiarum. But even before that, Pope Pius XII wrote, in his encyclical Orientalis Ecclesiae:
25. As it was in those times long past, Venerable Brethren, so will it be also today. More effective than anything else for promoting that reunion of all our separated sons with the one Church of Christ for which all good men are striving, will be a sincere and practical goodwill, with the help and inspiration of God. The fruit of such goodwill is mutual understanding, an understanding which Our Predecessors have sought so earnestly to foster and increase by various means, in particular by founding in Rome the Pontifical Institute of Higher Oriental Studies.
26. This goodwill implies also a proper respect for those traditions which are the special heritage of the peoples of the East, whether these be concerned with the sacred liturgy and the hierarchical Orders or with other observances of the Christian life, so long as they are in keeping with the true faith and with the moral law[emphasis mine-ed]. Each and every nation of Oriental rite must have its rightful freedom in all that is bound up with its own history and its own genius and character, saving always the truth and integrity of the doctrine of Jesus Christ.
27. We would have this to be known and appreciated by all, both by those who were born within the bosom of the Catholic Church, and by those who are wafted towards her, as it were, on the wings of yearning and desire. The latter especially should have full assurance that they will never be forced to abandon their legitimate rites or to exchange their own venerable and traditional customs for Latin rites and customs. All these are to be held in equal esteem and equal honor, for they adorn the common Mother Church with a royal garment of many colors. Indeed this variety of rites and customs, preserving inviolate what is most ancient and most valuable in each, presents no obstacle to a true and genuine unity. It is especially in these times of ours, when the strife and discord of war have estranged men's hearts from one another nearly all the world over, that all must be impelled by the stimulus of Christian charity to promote union in Christ and through Christ by every means in their power.
28. But the work of faith and charity would remain incomplete and powerless to establish unity firmly in Christ Jesus, unless it rested upon that unshaken rock upon which the Church is divinely founded, that is, upon the supreme authority of Peter and his Successors.
In Anglicanorum Coetibus, Pope Benedict has applied these principles to the patrimony of Anglican Christians, recognizing that there is great value in many of the liturgical and spiritual customs found in Anglicanism. That even the Complementary Norms are not specific about this patrimony allows freedom for this distinct Anglican patrimony to be defined by the ordinariates that will be erected.