That's certainly something for the Club to keep in mind and it's a great suggestion. However, I'm not sure about the practicality of it as it does introduce a complication that will make administration very difficult or perhaps impossible. I'm trying to think of a class where such a rule is in place. Lasers do have 3 different official rigs (and a couple of established unofficial rigs), but these are all self regulating. For example you rarely see 50kg sailors sailing a Laser Standard, nor do you see 90kg sailors in 4.7s and there are no rules precluding anyone from competing in any category by reason of their weight.
If someone turns up to a regatta or club race and they are just under the weight, do they have to stand on a scale before the race and do you disqualify them because they don't have the correct rig? Bathroom scales are not calibrated, so it means the club has to invest in a proper weighing device.
Another way to go is to introduce corrector weights on the hull, but again, that's yet another complication requiring lots of administration.
I've just remembered one class that has crew weight restrictions. The International Star.
A recent windsurfer class (Kona One) has a system where rig size is related to sailor weight. As you mention, it's hard if the sailor's weight changes.
It's not hard, however, to put corrector weights in - Tasars do it regularly. The sailors step on the scales, someone looks at the dial and notes the weight on a list, and some sailors have to go and get weight which is tied in the boat.
It's not a big problem but also understandable if you choose not to go that way.
By the way, there is of course an 8' scow with excellent pedigree around for testing just a few ks from CRSC; http://www.nsc.asn.au/assets/publications/Northbridge_Junior_Plans.pdf
It's a very small boat but then plenty of American adults sail Sabots or Cape Cod Frostys.