I was playing around with some of my Tom Bihn bags and other accessories, and thought I'd note some of the possibly lesser-known combinations that I've found.
First, the Camera I/O fits perfectly within a Pelican 1500 hard case:
And here are some others, though without pictures:
As I mentioned elsewhere, the Smart Alec's Lower Modular Pocket clips into the annex loops in a Brain Bag, or a Smart Alec.
The Upper Modular Pocket, when empty, fits within the Lower Modular Pocket.
The internal dimensions of the Upper Modular Pocket are nearly identical to a Size 2 Stuff Sack, and so you can imagine that one of those fits beautifully there.
The Maxpedition Fatty Organizer fits easily within the Lower Modular Pocket. It will even fits inside the Upper Modular Pocket!
A travel tray folded down fits nicely within an Upper Modular Pocket.
The Co-pilot fits easily into the Brain Bag's first compartment, either horizontally or vertically.
Lots of things besides cameras will fit in a Camera I/O! It effectively adds a padded compartment to your Brain Bag.
3D Organizer cubes fit perfectly inside the "kangaroo pouch" found in the Smart Alec. This is now the dedicated location for my emergency medical kit, tethered by a 16" strap of the same color so that it's easy to pull out when needed.
3D Organizer cubes also fit very nicely into either of the side pockets of the Co-Pilot.
Small organizer pouches fits nicely into the two pockets inside the larger compartment of the Co-pilot.
5" flashlights, like my Eagletac P20C2, sit comfortably either at the bottom of the organizer pocket on the Smart Alec, or in one of the inner pockets of the Upper Modular Pocket.
The Hydroflask 24oz steel bottle fits in the Smart Alec's water bottle pocket as if it was made for it. It also fits nicely in the outside water bottle holder of the Brain Bag.
Have others found any interesting combinations?