Hi Tim,
The MacBook fits in a Size 6 Brain Cell, so yes, you would be able to fit it horizontally into the Brain Bag as BJ Nemeth does. The ordering page for the Brain Bag has drop down option lists for additions, and only lists the Brain Cell options (Horizontal or Vertical) that fit in the Bag. (You do first need to check the Laptop Fit List link on the page to find out which size Brain Cell works for your laptop, though).
Another accessory that's sized particularly well to work with the Brain Bag (and Brain Cell) is the
Snake Charmer. (I have one of these even though I don't have a Brain Bag). With the horizontal Brain Cell in place, it conveniently sits on top, for easy access to cables, etc Here's a link to a
review of the Brain Bag on a recent blog that describes the experience of using this with the Snake Charmer, Horizontal Brain Cell, and
Vertical Freudian Slip as accessories. This may give you a better feeling for what choices might be useful for you and how other people are using these bags with accessory combinations.
In the meantime, enjoy all your new Tom Bihn Bags.