I suspect that this is a little off-topic for this list, but I presume
(perhaps foolishly) that the whole point of using URIs as namespaces
was precisely so one could choose a namespace that *is* under one's
control.

No one else in the world is going to use anything in the
'www.wwp.brown.edu' domain for a namespace except me. Similarly, if
you've paid gmx.net for your web-hosting services, the namespace
'http://ns.gmx.net/~noisyb/project1' (or whatever) seems perfectly
safe from other's use (until such time as you end your arrangement
with gmx.net at least).

On the other hand, lots of people *might* decide (foolishly, I would
say) to use localhost.

I don't know what, if any, legal recourse one has to protecting a
namespace, but boy I hope it never comes down to that.