A BOB Yak weighs about 14 lbs, only a few pounds heavier than 4 decent panniers. Unless you're really good at design and fabricating, a commercial trailer will almost certainly be worth the investment. You'll be able to haul exactly the type of backpack you want, without jury-rigging up something weird, unbalanced and/or inconvenient. You already have the willpower to cut out the fat, so it's unlikely you'll just pile crap on the trailer. Worst case scenario you can get one from fleabay for $200 or so.

Frugality has its advantages, but if you're investing the time and energy into a tour as extensive as this, I see no reason to skimp on something as critical as your luggage system.