this isn't quite the same thing, but this is what we use (we just started EC, but we've been using this to save the mattress since we use cloth dipes and sometimes have leaks).
i found a wool blanket at a thrift store, and felted it by washing it in hot water and drying it in the dryer. then i lanolized it (i dissolved some lansinoh/purelan creme in a bucket of warm water and soaked it, then wrun it out and hung it to dry). it is absorbant a bit, but also very, very waterproof. it works great for us, and was practically free. just like wool diaper covers, you don't need to wash it every time it gets peed on- doesn't smell a bit. only have to wash it occasionally, we don't have leaks very often anyways. i call it a puddle pad?
i haven't seen the flannel/PUL ones before. good luck!
