Those who belong to the Church are subject to the Roman Pontiff.
In that case, I think the context within Unam Sanctam itself rules out your interpretation of the issue, for the simple reason that Pope Boniface VIII is specifically talking about subjection in the sense that the “Greeks” [the Orthodox] aren’t subject to the Pope, either back then or today. This is evidenced here:
“Therefore, of the one and only Church there is one body and one head, not two heads like a monster; that is, Christ and the Vicar of Christ, Peter and the successor of Peter, since the Lord speaking to Peter Himself said: ‘
Feed my sheep ‘ [Jn 21:17], meaning, my sheep in general, not these, nor those in particular, whence we understand that He entrusted all to him [Peter].
Therefore, if the Greeks or others should say that they are not confided to Peter and to his successors, they must confess not being the sheep of Christ, since Our Lord says in John ‘
there is one sheepfold and one shepherd”
This is quite clear and rules out any interpretation that the Orthodox today are subject to the Bishop of Rome in any way whatsoever. I get that you want to say that there could be this “invicible ignorance”, which is fine I think under the Orthodox view, but I think it’s ruled out under the Roman Catholic view given this bull.
So again, in what sense are the Orthodox not subject to the Bishop of Rome? Well, in the sense that Unam Sanctam says that they’re not subject but must be for salvation and in the sense that the Orthodox position entails that they’re not subject: they must be “confided to Peter and to his successors” and the Orthodox necessarily aren’t confideded to Peter and they say they’re not confided to Peter, which of course isn’t a verbal thing, but a position that you hold by being Orthodox.
This to me seems like the nail in the coffin since it’s clear that Unam Sanctam is talking about subjection in the sense of
communion with the Bishop of Rome, and not of the invincibly ignorant kind, if there’s even such an animal.