Update: I now have more than just a stub plugin in a simple C2 constructed 20x15 grid.
What I've now got is a PhotonPlayer plugin that's a modified Sprite plugin (ideally, it would be a PhotonPlayer behavior that could be added to a Sprite).
It dynamically loads the PhotonPeer.Lite JavaScript interface to the Photon server running locally, draws the PhotonPlayer sprite on the screen (movable by arrow keys) and sets up buttons for 'connect', 'join', 'disconnect', 'leave' actions, which enable it to join a game in a game lobby. That bit works.
To do next (the real multiplayer bit): enhance the plugin to send a PhotonPlayer's position via the server to another C2 grid in another browser and have it appear and move there (along with the local PhotonPlayer object), and vice versa.
Questions/suggestions welcome!