If you can make contact with someone on the fisheries side of Yorkshire Water,
http://www.yorkshirewater.com/recreation/?OBH=3744 your position sounds similar to Thruscross reservoir, which they have run as a brown trout fishery for the last few years. I've not personally fished it, but I know the background to how they have tried to develop it as a 'brown only' water.
Do the close season by-laws apply to stocked stillwaters for browns? I know the coarse season on stillwaters is different from rivers; has anyone followed this up?
If you have club members eager to extend their season, is there another lake under your control which could be stocked with rainbows with less detriment? I think it would be a shame to lose a 'brown only' water, as they are few and far between.