So far:
* there was and is CAL. That is offline only processor but it has features possible in offline only (I mean some functionality is impossible in real-time one track oriented MFX/ MIDI VST frameworks): there was more developers, there is less now.
* there was and is MFX. The API is open source, with examples. Not all (I mean not even all essential...) tricks are explained, but for some use cases everything is rather clean. Unlike CAL, that can be used "on the fly". Unlike CAL, some offline, especially multi-track, processing is not possible. There was some developers, there is no one now.
Some users have claimed, the problem is in C++... and requested some binding. One user has requested Lua binding. I have written it... so far no one tried it, not even the person who has requested it!
* VST and MFX was established as a DAW independent possibility to extend MIDI/Audio processing functionality, available for everyone. How many VSTs from Cakewalk community was developed? I can remember one...
* Control Surface API is open source since very long time. It does not allow to control the content of the project (at least not directly), but it allows to control Sonar. There was just several people which have tried to use that, there are still just several people.
* Sonar customization was developed long before it become official.
* every interfere with (close to) real-time DAW processing (even MIDI, even Surface), made by not experienced programmers, is an easy way to make Sonar unstable, up to crashes and freezes. It is not an easy job to make a bullet prove protection against misbehavior of user scripts in general (in some cases simpler then in other... I have implemented such protection in the Lua MFX).
And so there are just several people which really want to develop something, only subset of them which really can. Who was serious, has achieved some results (CAL scripts, MFXes, Control Surfaces, customization).
So IMHO: an idea to invest huge effort just to attract a bunch of noobs by the fact they cat mess a bit with Sonar internals is pointless.