Sounds like a good plan - I'd guess it's a case of providing some JS on
joind.in for a 3rd-party website to <script> include, and then have the 3rd-party run a function defined in that <script> (probably passing in some relevant parameters to the function, eg the id of a tag to draw into). If you want it to look different on 3rd-party websites depending on whether the browser is logged-in to
joind.in then it'll need some thought about cross-domain logins, but I'm sure it's do-able (maybe having the JS include something from the
joind.in domain so that the user's
joind.in cookies and logged-in state can be analysed accordingly).
Bit of a brain-dump, but it all seems possible.
Kevin