Authentication: Fall back from Mysql to LDAP?

Is it possible to fall back from one authorization scheme to another?

I have a database of users with passwords. What I'd like to add is
optional use of the users' Windows (read LDAP) passwords.

What I'd like to happen:
1. User logs into website
2. If (hashed) password matches that in database, then authenticate.
3. If password in database, is "USE-LDAP", try the entered password
for LDAP login. If successful, authenticate.
4. Otherwise, go to registration process, where the user can enter a
application-specific password or choose to use the Windows/LDAP
password.

I have two ways to check the LDAP password (mod-auth-LDAP or a simple
perl script), but don't know how to do the conditional fallback like
this. Does anyone here know?

Thanks!
--Buck