James,
At some airports in Europe, private businesses provide a service to "Saran Wrap" luggage with a fluorescent green wrap (I saw that at Milan / MXP recently - not sure what they charge for that service?). What type of Backpack are you using? If it doesn't have a "stowable harness" (which is provided on some Eagle Creek models), it would be a good idea to secure it in some way.
Some people use a folding Duffle bag for that, but I've also seen travellers use a Glad bag to enclose their packs. Some airlines will place Backpacks into a plastic "bin" so that it can move freely along the conveyor systems. The best option is to secure the straps in some way, if possible.
I normally lock my Backpack with a TSA-approved Lock, but disposable plastic luggage ties could also be used. I prefer the TSA Locks that have a small "flag" that indicates whether the bag has been opened.
Cheers!