John, I have done a roll down top closure that works well. Here is a link to pics...
roll top pack
As suggested I simply used 3/4" gross grain (stiffer type) that was run through a large draw string type chamber at the top of the extension collar. I let the gross grain come out on the sides and sew in a 3/4" buckle. To close it I simply fold over the gross grain just like you do a dry bag and buckle it.
Now for the field report. The roll top closure performed really well. It closed well and stayed closed, but it did not hold the top compression as tight as I wanted. I'm not sure any dry bag can do this really well. In looking at the new BPL pack I suspect its roll top will top compress better. Take a look at how the top straps come down the side, not buckle across the top.
I do think this type of closure will perform better at keeping out light rain. Many traditional UL packs have areas that can "pool" water when the collar isnt full. This design removes this. Field testing did not include much inclimant weather so this is untested (it never rains when you want it to).
I am currently redesigning this pack with a focus on how to improve pack opening and closing. I think I have a way to do it, I just need to test it out.