My parents get 50+lbs of rhubarb per year off their patch and my Mum freezes loads of it uncooked. She chops it into bits and puts 1lb in a bag with 1oz of sugar. It goes quite watery when it's defrosted, so not ideal for pie or crumble as it makes it pretty soggy, but perfectly good for stewed rhubarb, rhubarb fool or jam/chutney etc.
HTH 