Polypropylene Paver Base Panels
 

Has anyone used in yard under mulch to help prevent weeds?

Has it worked?

IMHO, there is nothing you can put in to prevent weeds, over the seasons (years) dirt will accumulate above the barrier, and you will get weeds. I use a good liquid weed spray and go out and spend 15 to 20 minutes of quality time spraying the weeds. One other thought, if I understand what you are proposing you will accumulate water during heavy rains, and you will not be able to water plants correctly.