This would probably be better in Michigan's forum, but I'll tell ya what I think.
The AG opinion on not being brandishing if you have a pistol in a holster isn't law, it's an AG opionion. There are no laws that prohibit carrying a long gun in a reasonable and non threatening manner, at least not that I know of. I am not a lawyer, but to the best of my knowledge, it would be totally legal if you avoid making any motion that would fall under the definition of brandishing. Certainly hunters are not arrested and charged with brandishing, so I know of no reason why you could be charged with that. But certainly cops have arrested and otherwise harrassed a lot of people here regardless of laws.
In a rural area, I think it's a great idea. You might run into all kinds of things that you'd want a long gun for. In an urban area, I think it would be terrible. Concerns are retention, over penetration, deploying the weapon in a building in a fast and maneuverable manner, and public image. The public image thing is especially important in Michigan and the other states where the issue is being fought, because the public tends to be more intimidated by big guns than by sidearms. It's critically important that the public understand and respect us, and that we do the safe and reasonable thing.
Having said that, as I've said before on this issue, if you research long gun carry heavily, and decide you still want to do it, I won't say anything more to discourage you from doing it in what you find to be a safe and legal manner. A right unexercized is a right lost, and if you want to use the right to carry a long gun, that's fine by me. Just please be careful and prudent.