Good to know you're liking your US-10 and 20 packs. And I hear what you're saying about backpacks being ideal for your kind of travel. Not to get all spammy or anything, but you might want to take a look at our R series packs (specifically, the waterproof R30) - the harness system is unique and removes any stress from the shoulders / neck area. Plus, you can quickly strap your US pack (which will have come with an included shoulder strap) on to any backpack for carrying off the bike. That said, we might well be adding a carry handle to the US packs in future. In the meantime there's a solid workaround using one of our Haul Loops that I'd be happy to share.
I have the non waterproof R-25 and whilts I've never actually had it soak through, I was wondering would it be wise/ok to treat it with nixwax cotton proof or textile spray to make it even more waterproof?