I think it's true that stallions may be more prone to cresty necks for hormonal reasons, but also that it is still a metabolic indicator just like in any other horse. There's no muscle in that spot so a crest in any horse is fat filling in the fibrous tissue that is there.
As far as weighing hay goes - I can only weigh my nets when I fill them and I use big nets because I don't get to the barn every day. I have the 3/4" hole, half bale and full bale versions of the ones Kelly linked above and they have been great, slowed him down considerably vs. the 1" holes. So he basically has free choice hay, but there are only so many hours in the day and since he has to painstakingly free it from jail before he can eat it, it does reduce his consumption.

And I don't stress that he is without hay, but I know from weighing them that on average he is eating in the ballpark of what is reasonable for him to maintain a decent weight.