gentoo's package.provided equivalent?

[anatoly at kazanfieldhockey.ru]

Sometimes i need to build specific version of some library from source,
not from ports. How can I tell port system about it. There was no problem
with old pkg_, because I was able just to put dummy directory into
/var/db/pkg and port system knows it is installed (although maybe that was
ugly hack). But what I can do with pkgng now? For example, Gentoo linux
has /etc/portage/profile/package.provided file for such a situation. Is
there some equivalent?