The kdbuswreck

> The problem is that I have yet to see anyone suggesting how a solution to the problem could look like.

What is "the problem" - specifically? Once you have that clearly stated, the solution probably isn't far away (and it probably involves file descriptors - most good solutions do).

> Maybe even one that could be initialized through the capabilities API so there is a graceful fallback?

I think the capabilities API could be part of the problem, not part of the solution.

Consider Superman. He can jump without flying, can look without burning holes, can listen without hearing every mouse's footstep. His super-powers only take effect when he wants them too.
If "capabilities" don't need to be explicitly activated every time they are used, then they are really just "defaults". And default super-powers can cause a mess.