1) Configure all of them to the least common denominator. Thus all can support the same passwords. This can be crippling if you have some system that is really limited in its support.
2) Disable password policies in the connected systems, but also block the ability to set passwords in them, requiring a password change via IDM (User App perhaps, or any app you allow it to happen in), This way your stronger password strength is enforced by a stronger policy, and they cannot change the password on their own.
3) If you are using Secure Login, only allow logins via NSL, and make the password for those apps ridiculous, such that no one could ever remember to type it, and thus they never know what it is to be able to change it. (Also make sure NSL is NOT allowed to change that password either). Every time they change password somewhere, regenerate that ridiculous password into their Secret Store to keep it fresh.