I can download the file using Tor Browser so it should work.
For curl to work you can try to use in curl the same headers/user agent/etc as in browser.
Apparently this whole discussion is a classical case of “should have RTFM” … man qvm-service reads among many other things:
qubes-updates-proxy
Default: enabled in NetVM
Provide proxy service, which allow access only to yum repos. Filtering is
done based on URLs, so it shouldn't be used as leak control (pretty easy
to bypass), but is enough to prevent some erroneous user actions.
… so anything NOT repos is apparently filtered and the whole endeavor to force downloading though the update proxy hopeless … switching strategy.
Thanks @apparatus for sticking through this with me.