This induces an overview file which contains 2 lines, because
somewhere along the way Gnus converts the carriage return to a
newline.  The 2nd line makes for the malformed overview entry, because
it begins with that `>'.

I would guess that what's needed for this is a patch to turn carriage
returns into bland space characters, the way that I expect all headers
have tabs converted to spaces to prevent screwing up the tab-separated
overview files.  Does anyone know precisely where this occurs?  A
quick peek at nnheader.el wasn't terribly informative.