I'm sure I read the start where you finish rule somewhere that made it look official, but certainly the evidence seems to suggest it never existed. Up to now I've not had the impression that any transfers have been done to take advantage of tailwinds. However, with a strong south-westerly this morning veering to north-west in the afternoon, today's effort seems to have crossed that boundary. But as you say, it could be coincidence.
I've always felt that if there's going to be a no drafting rule, it really needs to be accompanied by a rule that prevents motor-assisted tailwind chasing. I think that "start where you finish" is unnecessarily restrictive; it would be sufficient to require that all qualifying miles form part of a continuous loop or out-and-back route.