SeaBIOS is an independent project from QEMU. Currently, it is included in the qemu-kvm package, which makes it difficult to patch. Gentoo has a sys-apps/seabios package in the portage tree, so qemu-kvm really should rely on that.
I added sys-apps/seabios with the plans to switch over. I'm already aware of this.
It uses it with qemu-kvm-0.15.1