"How do Jam2 users carry this?"
I have only used my BV 500 with my Jam2 once and have decided that the next time I'll go with a bigger, and heavier, pack. Basically, I stuffed my bag and down jacket in a plastic bag in the bottom. I then put the BV in vertically. I rolled my tent, rain-gear, dry clothing and such into 12" long cylinders and stuffed these down beside the BV. The rest of the stuff including stove and cook-pot went on top or in the rear pocket. I could barely get the top closed and I carried a plastic trash bag and rubber bands to keep water out if it rained. It didn't.
The pack did not ride as comfortably with the big BV in it as it does with the "weekend" version of the BV; it was sort of like a beer keg with shoulder straps. The whole mess of gear, food and a liter of water weighed over 30 pounds. The BV and ten days of food weighed about 18 lb by itself. As the trip wore on, I was able to move stuff into the BV (mostly cooking stuff) and the pack slowly became more comfortable.
IMO, the extra 1 3/4 lb of a larger pack is well worth the added comfort it provides in carrying the large BV. When I hiked the JMT last summer, I used a GoLite Quest with my big BV. It fit horizontally in the bottom of the pack and left more than enough room for the rest of my gear. If you do this, I recommend cutting a small piece of 3/8" foam to cushion the BV. Otherwise, sharp rocks can easily cut through the pack bottom where it is stretched tight over the plastic.
Perhaps others on this forum have a better solution to the Jam2/big-BV problem than the one I devised. If so, I am really interested in hearing about it.