"How does something like that work in Rome for a priest who's not attached to a particular church -- do you need to schedule something ahead of time or can you just walk into a church and find an available altar and start setting up?"
Well, here's the way it goes. Any priest may celebrate a Mass at St. Peter's Basilica. There are certain "popular" altars that require a reservation (namely, the tomb of Pope John XXIII, the "altare alla tomba" which is downstairs on the grotto level of St. Peter's and faces the niche of the Pallia, the Clementine Chapel whose altar is the closest in actual proximity to the relics of St. Peter, and the Hungarian Chapel which houses larger groups who may have gotten bumped from the Altare alla Tomba by someone higher up on the hierarchical food chain (do I sound bitter?)). But other than those, there are also roughly 25 altars set up for the celebration of private Masses. Don't ask to celebrate Mass at the main altar under the baldacchino; Only the Pope, or someone delegated by him, celebrates Mass there.
How does a Priest do this? He goes to the sacristy of St. Peter's sometime between 7 and 8am. If a reservation has already been made for one of the "primo" altars, then he reports to the desk to the right and informs the man sitting there that he has a reservation. If any altar will suffice, he doesn't have to go to the desk. In the sacristy he'll find all of the things necessary for Mass: vestments, a dressed chalice, and liturgical books in various languages (though each altar is equipped with a missal in Latin and Italian). An altar server will take the Priest to a free altar. On the way, they usually ask if you have a preference. If you have a favorite altar (St. Gregory, St. Leo, The Transfiguration, St. Pius X, Our Lady of the Column, St. Josephat, Sts. Simon & Jude, St. Thomas, etc.), let him know. Once he leads you to an altar, he'll usually make sure you're all set, and then leave you there.
Two things jump out at me at this point:
- If you have any people with you in the Basilica who will go to your Mass, tell them to wait in the Basilica by the passageway to the sacristy. The altar server who leads you tends to walk quickly, so tip your group off that, when they see you being led by the kid, be prepared to walk just as quickly behind you.
- Even though there are Masses being said for the general public each morning in the Basilica, that sacristy door will become a hangout for people who want a Mass in a language other than Italian. They will try to look at the cover of the missal you or the server is carrying, trying to catch a glimpse of what language books you took. If they see no books, they assume you're celebrating a mass in Italian or Latin (the languages of the books already at each altar - remember?). Some may ask you if they can follow you, but most do not. They simply follow you. Or, they wander around the perimeter of the Basilica, going from side altar to side altar, waiting until they hear a language they recognize. These side altars all have a container with small communion hosts, for any Mass attendees you have. But don't be surprised if someone shows up at your Mass well into the Eucharistic Prayer, but still expects you to give them Holy Communion. Ugh!
Once you're done with Mass, bring the books, the chalice, and the carrier containing the cruets and finger towel, back to the sacristy. They'll take the alb and chasuble from you and hang them up, as well as collect the cincture and toss the amice into a bin to be washed and pressed for tomorrow's private Masses. Before you leave, sign the register on the desk, and at least once during your stay, drop some Euro into the offering box nearby, which helps pay for the hosts and wine used at these Masses.
If you have some time, hang around the sacristy afterwards. Watch the reaction of the Vatican crowd as clergy come into the sacristy. Priests are, so to speak, a "dime a dozen"; the only way for the sacristy gang to acknowledge you is if you do something wrong. Monsignori? Not likely to impress the sacristans. A Bishop might get some sign of deference out of them (at the very least they have to act as if they're impressed), and so everyone is on the lookout for a ring or pectoral cross that is the tipoff. Really, with the obvious exception of the Holy Father himself, only Cardinals can get the altar boys and sacristy staff to act like someone important is present and needs to be taken care of. It's great to watch the group dynamics that go on in that sacristy; kind of the Church edition of "Wild Kingdom".
On a Sunday morning, a great thing to do is concelebrate the capitular Mass at 10:30am. There's a schola and a big ol' pipe organ, and being in the procession up the center aisle of St. Peter's, around the Confessio, up towards the Altar of the Chair is alone worth the effort. Simply show up in the sacristy about 10am.
I hope this answered the question. I know it brought back some great memories. It has always been a humbling and prayerful experience celebrating Holy Mass at St. Peter's Basilica, whether I was alone, with a Priest friend or two, or with a pilgrimage group. Such a privilege.