Arjan, yes you can measure the prebent of the mast also with a rope from the top and then charging it with your weight in order to get the mastcurve under sail-load.

But remember there will always be (and it ought to be also!) a difference between your luffcurve of the sail and the mastcurve.
Exacly this diffence creates the (so-called external) camber of the sail. In other words, this makes a (vertical) wing of your sail!!

So the sail is always rounder then the mast! But not too much ! (around 5 cm difference is common, I believe)

By the way, in windsurf sails there are even sails who don't have any internal-camber (curving the sail by the sewing of non-linear seams) and get their profile purely out of external camber.

Twist in windsurfing sails is the most important factor.