I have to say that I never have used the body weight rule and hadn't really heard of it until this web site. I sure it has value and is a useful tool, particularly in establishing a *minimum* but I'm just a big beleiver in free choice roughage, either grazing or hay.
Here are my own highly opinionated and biased views on the subject, informed by my own idoiosyncratic experience:
A stalled horse should have hay in front of them at all times, especially a horse that's normally used to getting some turn out. A horse should have something in their gut at all times for optimal health/lower colic risk. If you feed free choice hay and your horse gains too much weight, I would switch to a lower digestable nutrient hay before I would restrict the amount of hay.
When planning/managing diets for horses, I feed hay until they started leaving some over to the next feeding or playing with it/dragging it around the stall, then I back off one flake.
If it were me, with my horse stalled at a showgrounds for a period of time, I would want to have plenty of hay available to throw a flake time if the horse were bored, upset, fussy, etc., and some extra flakes to put in a bag to hang while I braided or groomed, and then some extra just for margin of errror.
I recognize that in some parts of the country hay is scarce and expensive, but rationing or limiting hay never has appealed to me from a horse management standpoint.