I emailed Lowepro today asking about a case/bag that might hold 4x5 film holders that would be "sliplock" compatible and strap onto the outside of their bags.
Here is the response:
We do make the non-padded Trekker DayPack II, which can attach to the
back exterior of the Photo Trekker AW and Photo Trekker AW II. It's
specifically designed for film holders. We've just not had enough
demand for a product specifically for film holders.
Maury Cohen, Product Specialist
1003 Gravenstein Highway North, Suite 200
Sebastopol, CA 95472
I was thinking of something that would hold 5 holders or so and fit nicely on the outside of their bags.
If you think you might want something like this, email the guy at firstname.lastname@example.org