Unix 'capabilities' (before Capsicum, iirc) were a different thing with the same name. Confusing. Unix actually does have a different construct that is like a classical capability: an open file descriptor that you can send to another process over a Unix domain socket.

FreeBSD got Capsicum only in 10.x and many projects started using them.

One interesting thing built on top of that is CloudABI:

https://nuxi.nl/ https://www.bsdcan.org/2015/schedule/attachments/330_2015-06...

I'm curious; do you have any details on exactly why the existing implementations are too complicated to be usable, and on whether this looks likely to be just some specific design flaws or inherent in the concept?

Not at the kernel level, is it?