You would have to tie each weight to the line individually. In the case of soft weights the size of the part you tie to could change causing the loss of that weight. With hard weights any rough edge on the weights could cut the line. You would also be pulling on the weights in a straight line making them less effective then a belt that could dig itself into a soft bottom.
I guess I was thinking to pull the integrated weight pouches, and tie the reel off to them. Most have a handle or other structure to grab when you go to drop the weights, and that I would tie the reel off on that. I agree it is two items to tie off versus one with a belt, but then the belt needs to be re-buckled so the weights don't slip off.