> > +      (license (license:non-copyleft "file://COPYING"
> > +                                     "See COPYING in the distribution.")))))  

Also, that URL is invalid.

It seems that I'm one of the few people that regularly uses X11 forwarding
to run different programs from different hosts on the same display, but the
"file" URLs are supposed to be:

file://hostname/path

If you mean localhost, then you can say file:///path, then the localhost is
implied.

It IS possible and recommended to have full file urls like "file://host1/path1"
and "file://host2/path2", but here we can't because we don't know the hostname.

At most, the invalid syntax "file://COPYING" would mean a file on host "COPYING"
with no name (?).