How does the quantity of salt change after the steaming stage when cooking pulao?
Dum is the steaming stage of the cooking. This is the stage where rice absorb water and the flavor. From my experience and what all our generations have been told on how to cook pulao/rice, you taste the water/broth just after adding rice. It should be a bit saltier than what you would normally like. This way the saltiness in rice will be just right when you finish the cooking. 



EDIT (with the comments from Stephie):

Rice absorbs water and with the water, some of the salt.

In the beginning, you are tasting the water : salt ratio. After cooking, you are tasting the (water + rice) : salt ratio which means you are getting something that tastes less salty.