This is possible, but it's not a simple task.
I'd start with writing new scheme's for the ISPConfig data to save the same data in LDAP as you would in MySQL and create some "replication" function so the LDAP data is always in sync with the data in MySQL.
From that point on you can start having services use LDAP Authentication and possibly start making MySQL data "slave" ..
I like ldap, but i fail to see the big advantage over the already existing system. I think it's too much work for what you'll eventually be able to do with it.
anyway, good luck!