PyErr_Restore doesn't seem to trigger exception chaining. But thanks for the tip about explicitly setting the traceback; not sure how I missed that, but now the StopIteration traceback is visible.

Minor point: The previous patch was setting the __context__ of the RuntimeError, whereas it ought to have been setting __cause__. Have corrected that.

So here, before I move further forward, is a new POC patch; I've removed the patches that rhettinger applied already, and fixed up tracebacks. So now it's a better-behaved POC, at least.