I want to drive to Hearst Lake (map here) and explore the area on foot, but I am unsure what wildlife precautions I should take at a high altitude. Do grizzly, black bears, mountain lions, elk have a range nearing the treeline? Am I more or less at risk at that altitude?
Animals are the least of your concerns at that altitude.
I spend a lot of time at that elevation just on the other side of the border from Montana in Canada. Your elevation is less of a factor for animal encounters than what the weather is. Go hiking in the heat of the day and you will not see another living thing out on those rocks, because every critter with fur is going to be hiding as deep into the cool dark woods as they can possibly get.
As for presence, I've watched bears climb right up and over cols just to get to the other side, Grizzlies will dig up an entire mountainside munching on roots, and they'll climb up onto the scree to turn over big rocks looking for juicy moths that may be hiding for the day in their shade. Mountain sheep and mountain goats spend a lot of time up there eating lichens and mosses, but they're super timid. Animals live up there, but they won't be out in the open when the sun is beating down, they'll be out in the early morning, late evening, night, or whenever the slopes are in shade.
As far as risks are concerned, you're at greater risk of falling over backwards while taking a selfie and cartwheeling back down the mountain than having a deadly encounter with an animal above the treeline.
Elk do go to high elevations in the summer, and apparently they're pretty dangerous in spring and fall: http://www.canadianparks.com/resources/letters/let-dangerouselk.html . The thing to do to avoid being hurt by one seems to be simply to keep your distance.
Black bears are rarely aggressive, but those that are habituated to humans may try to get your food, so they're a nuisance. To prevent problems, use a bear canister. Safety from grizzly bears is a topic that I'll leave to others.
Although mountain lions do sometimes kill people, such attacks are so rare that I can't believe it's worth worrying about.
In general, big wildlife tends to be fairly rare up near the tree line, because the ecosystem just isn't rich enough to support a lot of life.