What happens if you bring up an interface using an init script on gentoo? Do you know? Already figured out how to start services depending on what interface is up/down?

The gentoo-way is done in /etc/conf.d/net.

Looking at the net.example config you might find some functions named preup(), predown() postup(), postdown(), failup() and faildown() – those are hooks for events which you may use to call some programs or scripts. That way it is possible to do really fancy stuff.