Ben, I will say it's true that only about every other year lately am I hunting in below zero weather and snow. But I can tell you going up and down mountains in deep snow can load up a rifle with snow. And we certainly do hunt in rain and freezing rain. At least I do. As to not knowing what the UP is like, would you consider hunting upstate NY's Adirondack's to be a good comparison? How about coastal AK?
If you want a project, far be it from me to tell you you shouldn't. But I also have been at this long enough to know it's not necessary. It's a want, not a need, and is not without risk.
As to water not being able to find its way through from the safety slot or from the gap at the bottom of the bottom metal... all I can say is water has a mind of its own and does logic defying things. Also, a rifle is not a static thing in only one position during a day of hunting.
I'd be far more concerned about a typical closed trigger (like the Kimber's) than a BDL vs ADL magazine setup. To me, the ultimate is a blind magazine Model 70 with traditional open trigger.
My question to you is, what's your experience with the Kimber MT in the UP weather you describe that makes you think the blind magazine will be a problem?
Here's a Kimber MT and a little eating sized elk I took a pic of on a typical snowy hunt. My rifle looks like this as often as not on a snowy day.
-28*F