Sharing webapp2 session between GAE python modules with custom domain?

Question

I have a multi-module GAE app sharing the session info properly across modules when served via appspot.com. I used this recipe:

config['webapp2_extras.sessions'] = {
    'secret_key': 'my-super-secret-key',
     cookie_args': { 
         'domain' : ".appspot.com"
}

But session sharing no longer works when served through my custom domain (modules are mapped to different hostnames on that domain) - understandable since that domain is not known to webapp2.

If I update the config to match my custom domain instead session sharing no longer works between modules served by .appspot.com.

Is there a way to make session sharing work on both .appspot.com and my custom domain?

To clarify - I'm not talking about session sharing across domains, only within the same domain.

Thanks.