@MtxMan SSO can work with local developed apps. We have this running as an example with Atlassians confluence. Go in discussion with your developers and there requirements for authentication and test and test again. Getting SSO working will be too a process of development. There are a lot of options on the MOB to parse SSO requests and change something or not:
Another option will be to let MOB-portal store the users credentials for the local app. The user has to login once and MOB stores the password. This stored password will be reused everytime the local app will be accessed. Only a password change or if the stored password is deleted the user has to login again.