Hey, kungfu.
I went the opposite direction insofar as road bikes go. I picked up a Kona Jake-the-Snake and pulled off the Michelin knobblies and installed some
Axial Pros. After a few centuries and many miles with the local club, I'm now
running Panaracer Paselas on that bike. Unfortunately, the bike takes up too much space inside the minivan (which can't have anything noticeable on the outside... nature of my job) so I have the Montague.
As you can see, if you scroll back to my picture post, the folding Paratrooper has one thick-assed frame tube in place of the top-and-down tubes.
My rifle is a Marlin levergun, which is excellent for the purpose, and after much consideration, I'm looking at securing a scabbard parallel to this main tube. It'll protrude a little out front, but that's fine (I'll have a rack up there anyway).
I've put a lot of thought into your fork mount, and the only problem I'd have with that is all the extra weight on the steering assembly, but that's obviously been dealt with in the past with some success and I'd imagine it's more a matter of getting used to it than anything else.
I also considered leaving the rifle home, and relying on my XD. The bike's job, however, is to get me home over a considerable distance, and I'd feel more comfortable if I had a rifle handy, for the long term.
If, for whatever reason, I feel I shouldn't have a rifle on the bike (say, a societal breakdown which leaves a lot of law-enforcement intact) I may break it down and/or find a place to cache it before heading home.
Of course, ya can't do this sort of thing without some support for the bike, too. A Park roll-up toolkit, Topeak Morph pump (thing o' beauty. I didn't have a floor pump for nearly a year after I got my first Morph), tubes, lube, etc.
Then, of course, the Jym-supportin' stuff, like food, water, and such.
The list gets longer, but I'm sure most of you are familiar with the contents of a decent mobile survival kit.
Remember, even if you have a few hundred pounds to haul (for whatever reason), you can walk the bike with the load on it (say, a deer), without much more energy than it'd take to walk under a far smaller load. That's what the VC did in 'nam, and just ask the Frogs how well that worked out.