The proposal is interesting. I would take it to be a useful convention
for readability and a useful way to avoid clashes.

But some of the restrictions need unnecessary.

Who cares if there at dots in names? web:plt.org:extensions seems ok.

Why does :: need to be special? If a library foo:bar is registered, then
foo:bar:more is just as expression and clash free as foo:bar::more.

What is wrong with saying things like srfi-73x?

If URLs are allowed, why restrict to only http:. Why shouldn't ftp: be
allowed, or jar: to extract contents from an archive. Why not file: for
local libs, or for during development?

I am finding that Java's notion of reversing the order of domain names
in package names allows on to express namespaces "logical" scopes more
naturally. Consider:

  ace.com::tools:drawing

verses

  com:ace:tools:drawing

Myself I would have a very very simple rule: use Java's package naming
convention and be done with it. Your domain name gives the uniqueness.
Use a *single* separator by convention, either . or : or / or -, but
just stick with it. Note also that this does not prevent the use certain
prefixes with domain names since they are useful in their own right,
e.g. srfi-N, r6rs, etc. (Although I would prefer scheme:r6rs instead).
But when in doubt about uniqueness a domain prefx can be used.

Ray