Thinking about UE/Unity model more...
Ashley if you do take the big step and encapsulate everything except API in the future. There may be a business/revenue possibility for Scirra.
Add a license to get an unecapsulated version of C3. This license could explicitly state that there is no additional support for licensees that work with undocumented features / API. So, it will be very explicit about where the responsibility lies. Also it will _limit_ the number of devs who actually have this type of access to ones that accept the responsibility and also show their commitment via a payment.
Now to make this have real added value vs the situation we have today where addons can already use undocumented features and make it worth the extra license payment. With the above knowledge about licensees responsibility of using undocumented features and possible feature breakage. This additional license would also provide a C3 editor with code that is _not_ obfuscated. Which would allow advanced devs and addon makers to look at doing more advanced editor customization to improve their workflows.
So, you would then have a limited subset of devs with the resources to both do / hire out for more involved customization than the SDK allows, ability to pay for the extra license and explicit knowledge from the license agreement about the possibility that they may need to freeze their C3 editor and runtime version due to the customization they have done.
I actually like this idea, I feel like it solves the main issues at hand. Anyone willing to pay extra to tinker in the engine probably has the brain power to understand the ramifications. To pick up the car engine metaphor: A licenced mechanic can tinker with the engine and is expected to solve any issue that comes up alone.
Obviously this still adds probably a good bunch of extra work for scirra, but it's still a solid solution.