Actually, in one of the deleted scenes from Serenity, when they're making the ship up to look like a Reaver craft, Mal says to put Book up on the front of the ship, something about respect? My copy is @ my best friend's house, and I'm @ work, so I can't refer to it right now, but Book WAS one of the bodies they stuck on the ship. So I'd guess the markers are memorials, and not necessarily "final resting places".

If you'd like, I'll try to find the exact quote later tonight. Just reply to this if you're interested.