Thanks again. I checked out Roberts. Overall, it looks like a good store. A lot more than Ritz Camera, probably more than most local independents. (If your friend does a lot of business, suggest B+H Photo Video to him, in Manhattan. Good prices and good customer service.) They don't even sell large format, so I don't think tey can help me, though.
I've looked at hundreds of bags over the years, and owned probably more than a dozen (mostly cheapies), and I've never seen anything like what I'm describing. That's why I'm pretty sure it's got to be custom. Even the bags I've seen that were designed for large format operate the same as all of the others: basically, you open up a large flap that makes up one side of the bag, from top to bottom. This doesn't work well in many instances, like rain, or wind kicking up a lot of dust.
I really need something that has separate compartments that can be opened and closed individually, and are big enough to hold the 12x12x3 camera and several (say 10-20) 4x5 film holders.