There are two ways we can implement it - in the GUI only (make a login handhsake, obtain token, stream it down straight to the daemon via DBus - ugly and silica only.
Or we can do it right way - actually extend appstoreclient to do the login, add simple credentials entry dialog, add accountToken attribute to the pebble object, make it sync downwards via dbus to store in the libpebble/jskit. That way rokwork guys would just need to add their version of the simple credentials dialog. Worth checking with them though if they're planning to add this feature, it's quite an obvious miss.