I'd love a solution to this too. Having thoughts about how to mount a holster case vertically in the sort of position a bar bag would normally be. Take a look at the pics of my Raven Tour here http://www.thorncycles.co.uk/forums/index.php?topic=2026.0
Imagine being able to mount a holster case - sort of strapped to the front of the accessory bars, down across the handlebars, and then attached to the bottom accessory bar as well. That was one idea to play with.
Otherwise, frame bags were my next source of inspiration - could you mount a holster somewhere within the frame?
Alternatively the kind of slingshot bag you can swap from rear to front quickly - but I'd hate to cycle with one.
Surely someone's solved this before though??