Looking at dm-crypt wiki, i found an init-script (actually taken from the forums), see: http://www.saout.de/tikiwiki/tiki-index.php?page=gentoo-init-script what about including this init script in the device-mapper or cryptsetup package?
what about checking if there is already support for dm-crypt in the init system? :) this is included in recent versions of baselayout.