For now I think I'll work around it by simply skipping the corresponding tests and noting in the docs that you must have an IPv6-enabled Socket module to use this,

Sounds like the common practice :)

but it would be nice to be able top specify this in a Module::* installer.

Hmm, so a patch to Module::Depends - identify the dependencies of a distribution

and a patch to Module::Install#requires

So you end up with

 
requires qw' Socket 2  unpack_sockaddr_in6 ';
# requires qw' Socket 2 Socket::unpack_sockaddr_in6 ';
[download]

That could work, and doesn't look particularly hard to implement.