Well, ARC/ORC is a reference-counting concept for one part and also a new implementation for Nim. MPS is a mature and proven implementation of a whole memory management infrastructure which offers different GC strategies (M&S, Copying, Incremental, etc.). And it's written in C, so it could well fit with the Nim runtime. Here is a paper: https://www.ravenbrook.com/project/mps/doc/2002-01-30/ismm20.... MPS is surprisingly unknown. I also only discovered it recently and currently try to use it in a project.

EDIT: maybe a reason why people didn't consider it so far was that earlier versions were only available under Sleepycat/dual license; current versions are BSD.