Any contribution to existing Spring Social projects can always be done through the GitHub pull-request model. In short, fork the project, make the changes you want to make, and then submit a pull request. Take care to adhere to existing coding style and to include tests or else I'll be much slower to merge your pull request and may even send it back to you to clean up and write tests.
That said, it is possible to do what you suggest...at least to a point. Implicit signin is demonstrated in the Spring Social Quickstart samples and similarly demonstrated via canvas sign in in the Spring Social Canvas sample (your other issue with a 405 notwithstanding). The only thing those samples don't do that you speak of is to clean up the connection after the session is destroyed.
This is a good discussion to have, though. If you'd like to continue discussing how to create connections for on-the-fly users, may I recommend that you start a new thread on the subject so as not to clutter up this thread on how to contribute to the project?
Spring Social Project Lead