I recently read in Richard D. Nelson's Raising Up a Faithful Priest an interesting comment: "Priests served as tax collectors and assessors, determining the monetary worth of vowed animals and real estate (Lev27.12, 18) and the ability of individuals to pay the required amounts (v. 8)" (p. 47). While I realize it's probably not what Luke (or any other Gospel writer) referred to by the title "tax collector", I am reminded that Matthew was a tax collector and formerly named "Levi". Any significance?
One the other hand, the biggest problem with equating the tax collectors in GLuke with the temple establishment is that Pharisees rebuke Jesus for associating with them. Luke 5.27ff. tells of Levi the tax collector's call to follow Jesus. Subsequently, Jesus eats with the tax collectors and the Pharisees are upset. If these tax collectors are temple people, then the Pharisees would hardly be upset.