I did it Manitoba in 08' in exactly the way you described. It was one of the absolute funnest things I've ever done.
I did just as you mentioned... I set up a small natural blind - just enough to hide behind with a bigger tree as background. I kept gloves on to hide the brightness of my hands and wore face paint for the same reason. I sat on one of those small tripod seats with a back. Kind of important to have a seat with a back on it because you can only sit still just so long without being able to lean back and relax.
I was about 10-11 yards from the bait, downwind.
As far as the bears go, I never felt threatened by any of them except one smaller bear (small by Manitoba standards) that kept huffing, puffing and bluffing. He got to about 3 feet and I whispered something to him along the lines of "Hit the road", and he couldn't have jumped and run any faster. I have to admit the bear spray was ready to go on that guy. :p
I honestly believe that if you stay calm, don't make any sudden moves, you're fine. They know you're there whether you're on the ground or in a tree and they'll tolerate quite a bit. And really, if you think about it, if you run into a troublesome bear who's looking to give you a rough time, it's not going to be much more difficult for him to do it whether you're 12 feet up or on the ground.
I can't say I had any experience with sows and cubs on that particular hunt. I also think that to some extent a sow with cubs may at times avoid a bait site, knowing that boars frequent them. Not sure about that though... That's just my impression.