I doubt boxing will be possible, perhaps even be frowned upon for diablo3 by blizzard.

From a UI/client standpoint, too many problems would come from there being no lock on targeting, just soft targeting, IMO.

Also, with them saying Absolutely No Addons/Or Mods makes me lean towards they do not want anything running in the background interacting with their games. With the addition of the real money AH they are going to combat botting on a more agressive approach than thier other games, and while the debate of fairness to the multiboxing community, blizzard is probably going to take the approach of no external influence at all. While the option of hardware boxing would be much easier to impelement in that situation, with no mods or addons, you can almost state with a level of certainty that there is no macros or editable commands, at least in the beginning.

tl:dr - It is my personal opinion that boxing will not be possible at all. This of course is subject to change if any information comes out that would show otherwise.