Can I use ldapauth to connect our ldap users to drupal, and hence CiviCRM even though they don't have an email address in the Drupal users table?
Yes. The LDAP module for Drupal does map email addresses from the ldap directory to Drupal. It does have to be setup first though. In Drupal under Admin>>Settings>>ldap_integration>>ldap attributes specify the ldap attribute where the email address is stored for your ldap user.
In Apple's Open Directory the attribute is simply mail.
If your LDAP directory doesn't have email addresses in it (it's just serving Samba users), you can probably "fake" the email address by setting the samba user name (which is probably unique) to be the email address.