I don’t know why you would use the word “exposition” which in the liturgical context refers only the Blessed Sacrament. Since the creche is not a liturgical necessity in the first instance, there are not any rules other than the general norms for what may be in the sanctuary. It seems rather obvious that it should be located in a place convenient for people to see it, to come close and meditate on the mystery of the nativity, the Holy Family, and venerate the Christ Child, whereever that place may be in a particular church. There are probably good arguements for not having it in the sanctuary if it would impede the ministers during Mass, distract people from the Eucharist, etc.
This is precisely why when my PV served as our cathedral’s first rector, he moved the creche to the back chapel of the cathedral. It was much more spacious and it was beautiful. Because we had to have the necessary space for the cantor and altar servers, along with the placement of the temporary catherda (after seven years, we still don’t have a permanent one
) and seating for the rector and deacon, it was much better to move the creche to the back.
At my Dad’s parish, the sanctuary is not very big (church was built in 1897). Unfortunately, when you put the creche in a space that small (along with two big artificial trees
and lots of poinettias), there’s not enough room for anybody. The flutist and the cantor barely fit and the lector had to stand next to the flutist while the cantor sang the psalm (it was horrible, by the way; the psalm was set to “Go Tell it On the Mountain” and it sounded awful, that, and the fact that the pastor, who has an STD in liturgy and loves to brag about having concelebrated Mass with JPII, took it upon himself to replace the “Gloria” with "Hark, the Herald Angels Sing, made it even worse).
To disimo, save your ammo for a bona fide liturgical abuse, because even St. Peter’s Basilica doesn’t have the creche set up in the sanctuary. It is placed in one of the side chapels. Perhaps, next year, you may want to set up the Book of the Gospels in a prominent place and then right below it, place the statue of the Infant Jesus. They had that kind of a set up at St. Peter’s Basilica. After Midnight Mass, your pastor can then process to the location of the creche and place the Christ Child there. That’s what Pope Benedict XVI did after Midnight Mass. It was beautiful.
