I implemented a custom login (not LDAP) method by adjusting the login functions in core. I think it would be nice to allow two steps of customization:

1. allow a custom authentification function that verifies user+password and returns true/false (this is what I wanted)

2. allow a custom method of getting the userlist itself from an external system. I think this could be problematic, at least someone has to make a table which translates external user ids (e.g. username) to some mantis user id. but this could be done in the custom method (I don't know how this is done with LDAP).