Its almost there, but not quite. Here’s a usage scenario where its not working:
Multisite Network is created at mu-playground.com
The domain fish-playground.com should point to page mu-playground.com/fish/
When I add the domain fish-playground.com into the Domain Mapping plugin, mu-playground.com redirects to fish-playground.com.
When I add the redirect rule into DMS plugin, then the sites get stuck in a redirect loop.
… Not sure what steps to take next. But really, the thread is nearly resolved.