My guess is that all the established models were special requests. I think that agencies, accidentally or maybe even on purpose, call their models exclusives but they really are special requests. Cologne_rocks if what you are describing regarding Poly is true then I would consider her an exclusive as well, but just curious, considering the high rate that I assume Poly commands, do you think that Miu Miu would actually pay the opportunity cost to keep her off the market?
ETA:
I think with newer girls they are exclusives, but my guess is that, as a general rule, designers like Prada and Givenchy are not paying extra for the models to walk exclusively for them, rather the agency is charging normal and maybe even reduced rates and then using the prestige of their girl being a Prada or Givenchy exclusive to market their girl for the remainder of the season and afterwards.