PyOS_double_to_string is part of the stable ABI. I don't recall if we're allowed to add new bitfield flags to a stable ABI function. We'd use a new Py_DTSF_NORMALIZE_NEGATIVE_0 flag for this feature.
I suspect we can't add a flag, due to comparability reasons (new code setting the flag being used in old versions of python without it), but we'd need to research. I saw a similar discussion within the last few years, but of course now I can't find it. Maybe old versions would correctly ignore the new bit being set.
This proposal becomes less interesting if we'd need to add a new function to support it. Although I guess we could do something that's internal-only.