I've done all the bosses except 4 (mimiron, yogg, alganon, and freya) on PTR and imo they are not going to be multiboxable at release. The first boss isn't actually too bad, you have a few people driving vehicles and a few people riding on top of those vehicles, and they have to jump off and attack the boss when he gets certain vunerabilities. I think you could 2-3 box it as long as all your windows are visible at once. You do need to kite the boss around so its not like malygos or oculus where you all stay together; the "MT" debuffs/kites the boss and other guys attack, and he randomly will switch targets. You do have to clear a bit of trash before you fight it too.

It gets much worse after this boss however. For example on Hodir you have to split the raid into two parts, one fights in an arena while the other goes through a gauntlet type event before rejoining the first group. Then he is constantly throwing stuff on the ground, and depending on what he is doing you need to spread out, clump up, step on certain things, or be off other things.

I don't want to say it will be impossible to multibox, I'm sure someone will figure some clever stuff out, but I think its going to be very difficult. The "good" (?) news is that the instance is fairly difficult even in regular mode, and they have "hard" modes on most of the bosses that are very challenging, so its not going to be like Naxx where random pugs 1-shot everything imo. We shall see though!