Ive played through a couple promotions and it seems like points are maintained across promotions.
I know this wasn't always true, but it was on nov 17 (last promotion I had).
That said there is no good way to compare players who haven't faced off... Period.
Not across leagues, not across divisions, not within divisions. Points are a completely inaccurate measurement and rankings are based off of points. If we look at the blizzard top 200 (released) we see poor correlation with points. Blizzard uses hidden ranking for those numbers. If we compare that ranking to the ELO ranking that some fan sites use, we see an even different set of results.
The reality is that voting systems (game theory) require impractical amounts of data for a game like starcraft. The result being that you rarely ever get an accurate ranking system.