If you're running 1.5.2 or less, you'll need to modify the pwcheck daemon to chmod the socket after creating it. Admittedly, this program was not designed to do mass migration, and has some problems using it in this way. Sendmail's Cyrus support comes with support for forward files disabled. Sendmail can't mail to a file or pipe to a program address in the aliases file when using cyrusproto.mc. These are disabled by default for security reasons.
More information is on the cyradm man page.
It used to work, but since I enabled Cyrus mailer support, it doesn't. They're documented in the Sendmail documentation. Weird things are happening. More specifically, if you use NFS and it looks to work, you may have locking problems in the future which will result in silently lost mail. You need to either remove it from your system or modify the configure script to not look for it.