See also the wonderfully named SIGDANGER signal on AIX. It seems many people have invented this. I've been wondering out loud recently if PostgreSQL should attempt to support these various impending-doom-notification mechanisms, or whether a system that close to the edge really needs human intervention anyway.
The Amiga had a feature in the Exec kernel since version 3.0 that let applications register so-called memory handlers, which the system could call to purge e.g. thumbnail caches owned by the userland process, etc. Of course the Amiga had a single address space and no memory protection or paging, so things were easier.
Does this fix the cgroup memory reporting issue? It's caused me a lot of pain with containers containing crap code that assumes the memory is all available.