I agree it is a good idea to have some management of namespaces. I am not so convinced about the use of URLs for that purpose. (Apart from aesthetics, will `https://www.nieper-wisskirchen.de' and `http://www.nieper-wisskirchen.de' be different URLs for that purpose?)
If the library names become long (like with URIs), it would be nice each Scheme system allows to use or install a mapper (a simple association) from long names to short names so that I can access libraries as `(nieper foo)' instead of `(|https://www.nieper-wisskirchen.de%7C foo)'. If we go this route, we can drop URLs altogether and replace them with UUIDs, which everyone can generate (version 4 UUIDs). In fact, they are shorter than most URLs and no less pretty in library names.
A standard way to rewrite library names (in the same way that individual identifiers exported from libraries can be renamed in R7RS) would be useful.
Re: URIs and UUIDs, we just worked out this problem a few weeks ago with John in a different context: https://github.com/unisig/unisig. Our solution was to use either UUIDs, or URIs of the form "example.com/myname#fragment". I'd recommend that same form for Scheme libraries as well. Maybe drop the fragment part; we should think about it some more.
Gambit is tentatively using "@v" as the last part of the library name for versioning, where "v" is a Git ref (tag or commit).
I wonder if it's better to recommend that all URLs be enclosed in vertical bars, which is safe and simple as vertical bars are not valid in URLs unless %-escaped, or to specify that only URLs containing characters from "#[]'(),;" be enclosed, which minimizes the use of vertical bars.
Many R7RS systems don't define the mapping from library names to file names for library names that include characters like the slash or the dot. There may be other limitations.
It might make sense to have a convention to treat any library name with a dot as a URI by default. With the possibility for each user to manually add their own rewrite rules to override that default where needed.