Hello. Not sure if anyone offered any response to your question yet, but I'll offer my two cents. I'm not sure of the dimensions of the packs you mentioned, but I have a Zimmerbuilt Pack that is 8" deep, 13" wide and 21" tall (not counting the roll top). I was wondering the same thing as you: would a weekender fit horizontally in the pack.
The dimensions of my pack DO allow a glass "keg" that has a 9" diameter and is 13.5" long to ride horizontally in the bottom of my pack. I used the "keg" because that is what I had sitting right in front of me when I was trying to figure this out. The weekender having a 9" diameter and coming in at 10.5" tall would easily fit in the bottom of my pack. Not sure if this helps you, but you might be able to compare your pack's dimensions to mine to find your answer.
By the way, everything that I was planning to have contained in the main compartment of my pack did still fit with the "keg" in place. I'll be doing a thru hike of the Superior Hiking Trail to begin early May. Being it's still quite cold this time of year in MN, this means I'll be taking cold weather gear to include polar guard 3D insulating top and bottom and a 13 degree down bag. Also, going with a 3 pound 6 oz tent as well. I only added this so you could get an idea how much stuff I was packing inside the main compartment of my bag along with the Bearikade (that I'm still debating whether or not to buy).
Any advice as to whether or not going with the Bearikade is worth the extra $$$ over the BV500???