Has the Catholic Church responded to the apparent implausibility of a census as described in Luke? Over on History Stack Exchange, answers to the question Why would Jesus' parents travel to their birthplace for a Roman census? indicate that the scholarly consensus is that Luke is wrong in his descriptions of the census prior to Jesus' birth. Among their reasons are:


No Roman documents exist that indicate a census occurred for the entire Roman empire
Herod in Judaea, Quirinius in Syria, and Augustus in Rome did not rule at the same time
Requiring everyone to travel to one's ancestral home, especially one from 42 generations and 1,000 years before, is a logistical impossibility and we have no evidence that Roman censuses required this
Accounts of contemporary censuses, such as one in 104 A.D., required returning to their current residences (as opposed to being away on travel). Thus if censuses functioned similarly in Jesus' time, Mary and Joseph would have been legally obligated to remain in Nazareth rather than travel to Bethlehem.


Has the Catholic church provided an official response or explanation for the apparent implausibility of the census as described in the Gospel of Luke?
The matter of which census Luke referred to requires knowing the year Jesus was born. The Bible does not specify which year and problems with this are written about in the ‘Concise Sacramentum Mundi Encyclopedia of Theology’ (1981) edited by renowned Catholic Professor Carl Rahner. On page 732 Catholic scholars Ingrid Maisch and Anton Vogtle write:


  The year of Jesus’ birth is also uncertain. The difficulties can only
  be briefly indicated: Jesus is said to have been born under Herod the
  Great when Quirinius was governor of Syria (Lk 2:1). But there is no
  evidence that Quirinius was governor during Herod’s lifetime. None of
  the explanations of this contradiction so far suggested is
  satisfactory. All that is generally accepted is that Jesus was born
  before 4 B.C. (death of Herod). More precise details cannot be drawn
  from the infancy narratives of Matthew and Luke.


However, there are biblical clues about when John the Baptist was born, which is significant because John was six months older than Jesus. See http://taylormarshall.com/2012/12/yes-christ-was-really-born-on-december.html for Catholic reasons for a 25 December birth, based on their view that John the Baptist was born in the summer (argued for further down the article).  Some Protestants argue for him having a spring birth, meaning Jesus would have been born around September/October and conceived the previous December but either way, they fit in with the 7 B.C. census theory which now follows.

The year 7 B.C. was a rare one in having a triple conjunction of Jupiter and Saturn, which could fit in with the account of Magi from Persia seeing a significant star. David Hughes, Professor of Astronomy at Sheffield University, discovered that, three years before Herod's death, there was a rare series of alignments of Jupiter and Saturn. They came together three times over several months. The first conjunction was in May, he said, giving them plenty time to plot the next two conjunctions (in September and November) and travel the very long distance to Jerusalem, over which the second conjunction appeared at the time of Jesus' birth. Herod the Great summoned Jewish religious experts who told him and the Magi that a prophesied King would be born in Bethlehem. By the time the third conjunction happened, it brought the Magi to Bethlehem, to the very house where Mary and the child were. This implies a bit of a time lapse between Jesus' actual birth in a stable and being a child in a house in Bethlehem. Clearly, the stable accommodation had only been temporary (undoubtedly much to Mary’s relief). The Bible shows that Mary and Joseph stayed local as Jesus was circumcised on his eighth day in the temple, and Mary went to the temple for her purification.

Zoroastrian Magi viewed Jupiter as representing a new king; Saturn the old. The planets coming together would signify a change of ruler. This happening in Pisces would speak to them of Israel as they associated Pisces with Israel. This triple conjunction would make the planets involved appear to be travelling backwards and, on specific days, to have actually stopped. All planets do this, but it is very rare when two do so at the same time. This accounts for a special light appearing to stand still over Bethlehem in November that year. Thus goes the explanation according to Professor David Hughes. I have no idea if he is a Catholic or not, or even if he has any religious faith, but he claims his theory is based on cosmological events.  An article on this was in 1 September 2009 ‘Weekend’ magazine which was reviewing a BBC2 documentary on Christmas Eve that year. Now, here comes the link to Luke’s census.

There was a census in 7B.C., written about by 5th century historian Orosius. He said Augustus ordered a census of each province everywhere, with all men to be enrolled to show allegiance to Caesar Augustus. Josephus notes that "When all the people of the Jews gave assurance of this goodwill to Caesar, and to the king's government, these very men [the Jewish Pharisees] did not swear, being about six thousand." This census took place in 7 B.C. and was for non-Romans. Romans wouldn't need to be registered as showing allegiance to Caesar but Joseph would need to be registered, which accounts for him taking Mary to Bethlehem where he'd been born. That census mentioned by Orosius could have been that little known one Luke referred to, for Jewish men to swear allegiance to Augustus, which the Pharisees refused to do. It seems to have been for non-Romans, to boost Augustus's inflated ego and calm his nerves about disloyalty. The vanity of the Roman emperors knew no bounds and they would not care how much money was spent / lost in demanding people sign up to their rules. The wealth of Rome was fabulous. A census was nothing to them. Besides, it was the poor people forced to travel who would lose out and since when did Rome ever care about poor people?

http://en.wikipedia.org/wiki/Census_of_Quirinius supposes Luke’s account is wrong but then gives information on Quirinius’ later rule and the later census. Official Catholic views on this triple conjunction happening the same year as this little-known census mentioned by Orosius have not come to light in my research.