Put racks and panniers on it (you'll need those if you dont' have them already regardless of your choice) and go. The lack of braze-ons isn't too much of an obstacle. And a weekend tour will tell you a lot. You don't have to invest any money in upgrades to know how the bike handles with a load and where the problems might lie. Maybe you'll find it perfectly adequate, or maybe the load will reveal some fundamental weakness that rules it out as something you'd want to ride on a longer tour.

But it sounds like you've done the research you need already, so I wonder why not just go for it. You pretty much aren't going to go wrong with the Trek 520, or an LHT for that matter. I don't think there is any question that either will be "better" than the Bridgestone for loaded touring... as long as they really do fit you as well, or can be made to.