No it hasn't been done. You could do a cuben raft for gentle lake fishing if you used heavier cuben, but I don't think cuben would stand up very well to abrasion of rocks etc.
You could save a ton of weight off the Scout just by using lighter fabrics. Obviously durability would go down, but using 70D-200D heat sealable nylon would save a ton of weight. The Flytepacker raft is half the weight and it's 200D nylon.
That prototype 1.5 lbs raft on the Flytepacker site is pretty nuts. That'd be awesome for lake fishing. This raft is probably 70D or 100D nylon.