I tried it and there’s no tar warning and the file looks just fine,
including the long file names:

--8<---------------cut here---------------start------------->8---
$ make dist -j5
[…]
make[2]: Leaving directory '/home/ludo/src/guix'
tardir=guix-1.3.0.8612-7cad44-dirty && tar --format=ustar -chf - "$tardir" | eval GZIP= gzip --best -c >guix-1.3.0.8612-7cad44-dirty.tar.gz
make[1]: Leaving directory '/home/ludo/src/guix'
if test -d "guix-1.3.0.8612-7cad44-dirty"; then find "guix-1.3.0.8612-7cad44-dirty" -type d ! -perm -200 -exec chmod u+w {} ';' && rm -rf "guix-1.3.0.8612-7cad44-dirty" || { sleep 5 && rm -rf "guix-1.3.0.8612-7cad44-dirty"; }; else :; fi
$ tar tvf guix-1.3.0.8612-7cad44-dirty.tar.gz |grep patches/xplanet
-rw-r--r-- ludo/users     1252 2020-04-06 00:14 guix-1.3.0.8612-7cad44-dirty/gnu/packages/patches/xplanet-1.3.1-libimage_gif.c.patch
-rw-r--r-- ludo/users      765 2020-04-06 00:14 guix-1.3.0.8612-7cad44-dirty/gnu/packages/patches/xplanet-1.3.1-xpUtil-Add2017LeapSecond.cpp.patch
-rw-r--r-- ludo/users     6990 2020-04-06 00:14 guix-1.3.0.8612-7cad44-dirty/gnu/packages/patches/xplanet-1.3.1-cxx11-eof.patch
-rw-r--r-- ludo/users      783 2020-04-06 00:14 guix-1.3.0.8612-7cad44-dirty/gnu/packages/patches/xplanet-1.3.1-libdisplay_DisplayOutput.cpp.patch
--8<---------------cut here---------------end--------------->8---

> Seems a good thing to be as portable as can be, especially since 200
> chars patch file names wouldn't look good in the sources anyway ;-).

I don’t think we care about “portability” as understood in the context
of Automake, which roughly translates to “support SunOS 2 and HP-UX’s
1991 ‘tar’ implementation”.  :-)

Thanks,
Ludo’.