> you ask to modify Python so you can pass dict subclasses as namespaces and expect CPython to respect the mapping protocol

But no, per your own account, you made noticeable, though not complete, code changes in that direction. The only thing I'm saying "now that it's all there, it's only logical to fix the missing parts, about which people report issues". While you suddenly come up with "alternative" solution - throw it all away.

And I always just took your word for it, but now did some git logging/blaming. So, LOAD_GLOBAL handling of dict subclasses was introduced by you in https://github.com/python/cpython/commit/b0b224233e481d979430a54d257d871424ff19fb , in 2012, almost 8 years ago. 

And what about STORE_NAME handling? I traced that to https://github.com/python/cpython/commit/214b1c3aaea3e83302df9ea37a37b3c7548b92b1 of 2004.