I had the same idea three days ago and @ringabout pointed me towards [this patch](https://github.com/nim-works/nimskull/commit/419675a47ff322ecde59905aac427ffd8a298e99) which I ported to an unmodified Nim 1.6 and published it [here](https://github.com/Yepoleb/Nim/commit/c7a8f914f7e531a8aedd4ba007a9e931d7081e52) for you (removed the VCC compatibility though).
I was long mislead by the memory management documentation which claims "The reference counting operations (= "RC ops") do not use atomic instructions and do not have to -- instead entire subgraphs are moved between threads.", but this apparently does not happen automatically and atomics are still necessary for concurrent access, which happens often enought that it should not simply be ignored. Knowing which parts memory will get touched by a procedure is often incredibly hard to determine before running the code, so moving it all beforehand is not realistic. I am in strong support of your idea to add an option for this. Even if some consider atomic refcounts too slow to use for performance critical application, it's a great tool for prototyping without having to worry about manual memory management.
