I will take a look at the integration points raised in skymen's thread. Perhaps we can do some work towards that over the next year.
However making Construct in to a capable 3D engine is a huge amount of work - and likely endless work, regardless of whatever anyone claims about "we just need this one last thing". Providing APIs to integrate a 3D engine is also a lot of work, but probably not endless work, so while it seems a more feasible path, it's a lot of work either way. This will all take a lot of time and work no matter which path we choose, and that will require patience. Meanwhile there are other tools on the market that are already capable 3D engines, and if people really need a capable 3D engine to develop their game, then it may be wise to choose a different tool anyway, as it's not currently a goal of ours to develop Construct in to a fully capable 3D engine ourselves.
Accessing the engine internals against our advice is not a sustainable or responsible way to develop software, no matter what results it can achieve. Our SDK documentation clearly stated that undocumented features may be removed at any time. Presumably you had a plan for what to do if the features you were using were removed, as the warning stated may happen. If the addon SDK v2 removes those features, then you should implement your plan for what to do in that case. If you didn't have a plan, or assumed it wouldn't happen, then what can I say? We tried to warn you about this very situation happening. In the other thread the gist of what I was saying is that we will make a best effort to try to make sure addons can be ported where feasible, but that is not a guarantee, and we reserve the right to say "sorry, we warned you". In this case it looks like there's a lot of work whichever way we go, we do not currently intend to make Construct in to a capable 3D engine ourselves, and integrating a separate 3D engine looks like it might be a better way forwards in the long term, and that will inform our priorities when considering what to add to the SDK v2.