What's the best way to carry, say, two SLRs and a laptop with reasonably quick access to the cameras, good protection from weather and in case of a fall, and at least moderate theft protection so I don't have to unload everything and carry it into a bathroom with me?

Front wheel panniers look like the quickest-access option, but also the most likely to get squashed if something goes wrong. Does anybody make something like a lockable, weatherproof hardshell SLR holster that would work there? The weight of the cameras with the most commonly mounted lenses is under 4lbs each, so their weight shouldn't be a big issue.

I'd also like to be able to carry the lighter of the two SLRs for my regular day-to-day riding, possibly in some sort of handlebar carrier, but with the same weather and impact protection.

I guess the ideal solution would be the above lockable hardshell holster, capable of hanging either as a high-ride front pannier or a handlebar mount and able to be locked to the bike in either position.