phpBB and Elgg seamless SSO

Anyone has a working solution for seamless integration between Elgg and phpBB? I have poked around some forums, and was not pleased with proposed solutions, which involve including phpBB files in the third-party app and calling the phpBB functions directly. It seems phpBB has no REST or RPC API, but I may be mistaken. LDAP and/or OAuth remain an option, but those wouldn't be seamless. Any pointers would be appreciated.