Sounds like a good idea, like a little digital reading at the end of the pole which weighs whats in the net, may be difficult with larger fish but for some matches it would be perfect, you could tally it up as you go lol
Its simply delt with at the fishery, tell the anglers how many nets to use and make them split the fish. So if you are seeing winning weights over 120lb you say three nets etc... If the lad had have been told to use 3 nets and split them it would have been ok.
I think consideration needs to be given to the size of the net in use aswell. I bought a 3.5m long keepnet because I don't like to see fish crammed in to them for a long period of time, however, at many venues I have to stick to the same 50lb per net limit as other guys fishing with 3m or even 2.5m keepnets.
Maybe a weight limit dependant upon the size of net you are using would be more sensible. It wouldn't be difficult to police as it's easy to see how big a keepnet is, and this sort of rule would encourage anglers to buy the longer, more fish friendly nets.
I think the easiest way to get round it is by having a number of fish limit rather than weight.IE size of fish upto around 4lb. 20 per net.
Admitedly this only really works where you have a high stocking of similarly sized fish, but then you could also get round any bigger fish by the fishery having a bank runner who goes round with a pair of scales weighing anyone who feels they have a larger fish.
Our club has done this before, and it works well.
If the fishery is SERIOUS about their limits etc, they should be prepared to police it during the matches not just after them.
Then again, if you wanted to I suppose having a pair of stand on scales sat on your footplate could be an idea. Simply land the fish, and before you pop it into your keepnet just place it on the scales(while still in the landing net lol) at your feet for a quick weigh, add your weight to your tally and then to your keepnet.