Thanks! The 400 f/5.6 rides on the belt, I presume. That's a great combo, 400 5.6, 10-22, and 60D. I have been using 60D, 15-85 as a one-lens landscape/plant/general nature set up (note that in the Ozarks where I shoot, there aren't many grand vistas), and adding the 400 5.6 if I think I am going to have good birding. I thought the tripod might fit in the long pocket, as shown in the older reviews of the Mariposa, but now the Mariposa Plus is shown on the GG site with 2 short pockets each side. I suppose it might be possible to get the old long pocket substituted in by the manufacturer.
I put the 400 in a Think Tank lens changer pouch or Skin 75 on the belt. The skin is significantly lighter, but does not offer the protection of the pouch. The larger holsters for your camera will fit the camera with the lens attached in the pouch temporarily, but, as I recall, it protrudes enough that you can't zip it up. It shouldn't fall out if just walking around or standing, but it is not really a secure place to put it in for a long hike. I would test it for you, but the 400 I use belongs to a friend. We loan each other camera lens for such trips. As it is not my lens, I am more likely to use the pouch instead of the skin due to the pouches superior protection.
My only problem with using the belt system and the pack is when I have to to cross some creek in a wilderness area. Most of the time I find myself crossing using a downed tree and this can be a bit disconcerting even if you don't have a pack. But with a pack and camera gear it becomes a real problem that I have not totally solved. I bring along a large light weight water proof bag for my camera gear in in these circumstances, but the problem is what to do with the bag as you cross the river. It is best to save room in the pack for such situations. If you don't have enough room, which is usually the case for me on a trip over several days, then you have to carry the waterproof bag filled with gear separately. If you attach it to the pack it can swing around and totally screw with your balance. If you hand hold the bag, you lose a hand to grab onto something to maintain your balance. Usually, I don't hike alone in such situations and I have a friend go first and hand the camera bag over to them, but this only works well for very small crossings. The only other way is to throw a line across the river and use a carabiner to slid the bag along the line to the person on the other side. I have not had to do this yet and it comes with its own risks as I am not too confident of the strength of the attachment points for these light weight waterproof bags when carrying 5-6 pounds of camera gear. I have also thought about getting small waterproof bags, or even heavy duty zip lock bags, for each lens and the camera that will fit around the lens or camera while it is in the holster/pouch. However, whether this would survive a fall from a log into a river/creek is questionable and with the pouches/holsters on you become much wider which further complicates maneuvering through a downed tree across a river. So far, that is why I have stayed with putting the stuff in one bag and getting it across separately. Perhaps if I had better balance I would not be as anal about this.