sbuild has this shiny option, I want it too.
From sbuild(1):
--extra-package=package.deb
Make package.deb available for build-dependency resolution, by
adding it to a temporary archive created by sbuild. This makes
it easier to build packages against locally-built build depenen-
cies, without waiting for those packages to enter the main ar-
chive, or going through the hassle of maintaining a local ar-
chive and making it accessible inside the chroot. package.deb
is copied into the chroot, so it can refer to any path on the
host system.