Sorry that this is slightly OT, but maybe one should think
about this point in this discussion:

> It seems like USE would be an unconventional location to store that
> information and I'm not sure that it really belongs in the ebuild.

USE=live could perfectly make sense, if it is equipped with
the obvious meaning:

I suggest that if it is set, then it is attempted before building
to download the newest source from cvs/svn/git/monotone/bzr/...,
otherwise only the previously downloaded source is recompiled.
Currently, this functionality, which is extremely useful for
systems without permanent internet connection, is only
implemented inconsistently by using environment variables,
differently for each vcs:
  cvs: CVS_OFFLINE or CVS_OFFLINE_package_name
  git: EGIT_OFFLINE or ESCM_OFFLINE
  svn: ESVN_OFFLINE or ESCM_OFFLINE
  ???: Perhaps not implemented at all?

Then it would also make sense that @live-ebuilds consists
only of those packages for which the live USE-flag is
actually set (because the user does not want to treat them
as "live").