Would a hit from a high-powered modified blaster rifle (something a bounty hunter would use) possible have enough punch to knock an armored target (in AT-AT commander armor) who’s caught off guard off their feet and burn through their chestplate? If so, can the wound be life-threatening, or is there no such thing as a blaster rifle that can smash through AT-AT commander armor?

I am not aware of any source that says an AT-AT pilot’s armour is significantly sturdier than a stormtrooper’s, so yes, a direct hit from a high-powered blaster rifle could easily burn through a chestplate. That wound could certainly be life-threatening, but it’s just as likely that the pilot would be killed, especially from a close distance. We’ve seen such blaster rifles kill clone troopers from hundreds of feet away, and AT-AT pilot armour is said to resemble Phase II clone pilot armour. 

Hope that helps!

~ Jacen