I do understand your problem, but it would need custom coding to get it done. Currently, the LDAP synchronisation uses a provided tenant Id for all the users it finds in the LDAP server. The default one won't allow to change that.
In the upcoming 1.4 release, we've extracted the LDAP sync into an interface and abstract superclass you can use to match what you need: it will allow you (with a bit of custom code, not much) to set the tenant Id specifically for the user. In the upcoming 1.4 docs, this is documented in the section 'Custom Identity Synchronization'.