I’ll look in my commentary when I get home, but before checking that source I think we should all be able to agree that the canon
must mean more than simply assisting at worship carried out according to books used by Catholic churches. After all, it would be ludicrous to say that one had fulfilled one’s obligation by assisting at a service using the texts and rubrics of the TLM but conducted by a lesbian womynpriest. So the meaning of “catholic rite” that would more readily present itself than “rite used by, among other groups, Catholics” is “rite conducted by Catholics.” On that reading, an Orthodox Divine Liturgy fails the test.
Also, if one
can fulfill the obligation, one is bound to do so. But if you canvass the web for advice from respectable priests I’m pretty sure you would find most all of them saying you don’t have to attend Orthodox worship if no Catholic worship is available. From this it would follow that Orthodox worship does not satisfy the Sunday obligation, as an common-sense understanding of the canon would imply.
Now as to the original question, current law does allow attending Orthodox worship, though not receiving communion (unless it is physically or morally impossible to receive from a Catholic minister).* One always has to guard against scandal or indifferentism, but merely attending the liturgy, especially if done for the purpose of
experiencing the rite (sound familiar?) and not necessarily experiencing the worship of another communion, is permissible.
In fact, if a Catholic is not permitted to attend liturgies celebrated by ministers
not in communion with Rome (which is our nice cushy modern language for schisms whose borders are hard to define), then those who argue for attending SSPX Masses have no case. Actually, the Orthodox priests are in one respect in a slightly better situation than the SSPX because they stand under no canonical impediment and thus do not commit mortal sin by offering the Mass. Granted, their situation is worse from the standpoint that their schism is clear and thus they are accountable for that sin, but on the level of particulars they are not sinning every time they celebrate the holy mysteries.
*Can. 844 §1. Catholic ministers administer the sacraments licitly to Catholic members of the Christian faithful alone, who likewise receive them licitly from Catholic ministers alone, without prejudice to the prescripts of §§2, 3, and 4 of this canon, and
⇒ can. 861, §2.
§2. Whenever necessity requires it or true spiritual advantage suggests it, and provided that danger of error or of indifferentism is avoided, the Christian faithful for whom it is physically or morally impossible to approach a Catholic minister are permitted to receive the sacraments of penance, Eucharist, and anointing of the sick from non-Catholic ministers in whose Churches these sacraments are valid.
§3. Catholic ministers administer the sacraments of penance, Eucharist, and anointing of the sick licitly to members of Eastern Churches which do not have full communion with the Catholic Church if they seek such on their own accord and are properly disposed. This is also valid for members of other Churches which in the judgment of the Apostolic See are in the same condition in regard to the sacraments as these Eastern Churches.