StartTLS is a common option to encrypt communication between an LDAP client and the LDAP server. I know you support SSL but can not find any documentation on the StartTLS option for an LDAP server connection.

It isn't hard to add this functionality....So there is no reason you shouldn't support it in a future version.
Just have the option:

<securityProtocol>startTLS</securityProtocol>

And then in your client code detect the if this is set and make appropriate calls on the contect to start the TLS negotiation.
It is all built into the JNDI.