Have you installed
- cryptsetup
- cryptsetup-openrc?
That file should already be there, with included comments explaining how to set it up.
This information can be gained by running pkgfile(1):
$ pkgfile /etc/conf.d/dmcrypt
system/cryptsetup-openrc
On a side note, 66 supports /etc/crypttab through a variable in boot@system service configuration.