But I'm ignorant: is the Go runtime built in any way to accomodate third party languages? Apart from writing an interepreter in Go, is there a good way to target it?
No it's not. That's why you haven't seen many alternate languages.
Something else is that the semantics of the Go language itself are purposely limited in ways that help build an efficient runtime easily. So even if you could target the Go runtime easily, it would impose limits to what the language can do as I understand.
Go assembly is just a slightly funny version of the target platform's assembly. At that point, you might as well just assemble for the target directly and avoid the quirks of Go's assembler.
Something else is that the semantics of the Go language itself are purposely limited in ways that help build an efficient runtime easily. So even if you could target the Go runtime easily, it would impose limits to what the language can do as I understand.