I have a test case locally that demonstrates both the missing line
number in the __main__.py from a zipfile case, as well as the display of
main.c as the filename (the latter is not specific to the zipfile case -
it also happens for a normal main module).

It still seems to me that there is something going wrong at a deeper
level for doctest to be getting the "in main.c" part wrong. I want to
get to the bottom of that before working around the problem at the line
number lookup stage.