But that's still "draw the rest of the owl" category of complexity, and you probably can't really get hard real time on an ordinary kernel.

At that point you might as well take a look at runtimes that have a GC and is hard real time, which exists for the JVM. So not sure if it's really that valuable to separate out GC, especially when there are many other functions that can run much longer than expected (e.g. think of stack unwinding).