OTMM + OTDS 10.5 - Perform Login Using Externally Provided Credentials

Options

We have a custom web application that has access to OTDS user authentication credentials, and a request has been made to allow users of that application to seamlessly transition to OTMM using the existing knowledge of the users' credentials. The custom application already successfully makes use of various exposed otmmapi/otdsws services with these credentials. We were hoping to pre-authenticate the user via the web service api and simply include some form of verification in the initial request made when forwarding the user, but we have as yet been unsuccessful in attempts to PoC such a solution. Is there an existing combination of ticket/token/etc cookies that will allow OTMM to trust the incoming user without re-entry of credentials?