Fr. Christopher Danel, in Priest, Where Is Thy Mass (pp. 26-7) makes some interesting points that touch upon "permission". He states that
I was praying the traditional Breviary, and I made no bones about that.... I'd leave it sitting right there on the sacristy table with the pastor coming through back and forth. They really don't care too much about whether or how you say the office.
He also comments on the "Liturgy of the Hours", saying
It's so awkward saying the new Liturgy of the Hours because of certain "prayers of the faithful" that are added, the invocations, the hymns... They have all sorts of Protestant hymns in the new Office. Actually, since the hymns are so absurd, there are a lot of priests who just skip the hymn. They go straight from the beginning into the psalms, the way the Roman Breviary goes... Of course, there are those intercessions at Lauds and Vespers which sometimes are just atrocious, worthy of Communist propaganda.
So you see, in each quote there is evident a great absence of strictness in enforcing one pattern over another. This is a blessing. The priests themselves implicitly recognize the inferiority of the LotH, and overlook those who wish to use the earlier Breviary, as indeed some do. In addition to that their faith has been weakened to the point that they don't even pray the Office at all, in many cases, apparently. This fact adds to the apparent reality that the Novus Ordo generally depresses faith substantially. Which was probably the purpose its instigators had in mind.... So how could one need "permission" to pray the Breviarium Romanum?
All you need is Latin! And time.