There is not, and never has been, any enforcable means of claiming a whole root namespace; the demise of the registered modules list hasn't changed that at all.
Basically, just upload the thing, and document whether you want other people's extensions to live under the root namespace or somewhere else - e.g. DBIx::Class extensions are mostly just under DBIx::Class:: whereas Moose extensions live in MooseX:: - and a hybrid would be Catalyst, where certain types of extensions live under e.g. ::Plugin::, ::Model::, etc. and then things that don't fit in any of the usual extension namespaces go into CatalystX::
You'll find, generally, that people writing an extension to your code will respect your choices, and that any violation of them will usually be an accident and quickly resolved by filing a bug saying "hey, could you follow the policy please".
I did, honestly, mean to email you off list explaining what was likely to happen wrt the registered module list (starting a thread about it at the time on modules@ would likely have been ... unproductive).
Unfortunately, I completely forgot, a failure for which I'm hoping my responses to you today will at least partially compensate :)