There is no known reason why ISBoxer would not work with Classic. If it can be done, it will be supported. At the moment, there is no Classic to verify this on, but Blizzard have already indicated
Classic wont actually be running the original code, but rather the current code base but supporting the removal of features that came with the new code base, and based upon that, there is a great chance it will just work OOTB. We will really just need to wait and see when they release something, like a test realm of Classic, so we can test upon it.
As for the Addon system. Who knows on that one. Addons weren't***** supported with 1.12, but as they've indicated, there is a bit of LUA in the current version for some things (although these are specifically mentioned as server side), so it might come with the Addon system, but I suspect not if they really want to go down the recreation of an "authentic classic experience". That said. The Addons are not actually required, they just happen to make things easier.
/e
****** OK appears they were, but certain things just didn't work the same way, so the ISBoxer Addon doesn't work in 1.12