From a reader…
When Latin priests concelebrate at a Divine Liturgy, they are are to wear Latin Vestments (cf. Instruction for Applying The Liturgical Prescriptions of the Code of Canons of the Eastern Churches, #68).
The Deacon in the Divine Liturgy uses his Orarion for certain
actions, which are difficult or impossible to do with the Latin Deacon’s stole (Such as crossing the orarion over one’s shoulders).
Thus, what should occur when a Latin Deacon assists at a Byzantine Divine Liturgy?
I don’t think this has been answered by any official body. If it has, I haven’t see it.
The law says that a minister assisting at the liturgy of a different ritual church wear the vestments of his own proper ritual church. Thus, all things being equal, a Latin deacon should wear Latin diaconal vesture. Any ritual action which he is unable to do, such as crossing his orarion, he would simply omit, using the principle that no one is bound to the impossible (Regulae Iuris, #6).
I suppose if there is not any Latin vesture available for the Latin deacon, he could vest in the other rites vestments, but … it seems to me that with proper planning that wouldn’t be necessary.