With a different (larger shackle, but still much smaller/lighter that a typical U-lock) padlock one might lock the chainwheel to the right chainstay. This 'locks the cranks', but the bike could still be rolled away (freewheeled) forwards.

Upper bound: with my Rockymounts Carlito U-lock I can lock the frame/rear wheel/crank, and it only weighs ~360g.