If you have access to the app id and the app secret but no access to the app within Facebook it is possible to discover the creator_uid and from there the email address of the app owner using the graph api.
First get an app access token:
https://graph.facebook.com/oauth/access_token?client_id={APP_ID}&client_secret={APP_SECRET}&grant_type=client_credentials
With this you can get the creator_uid:
https://graph.facebook.com/{APP_ID}?access_token={APP_ACCESS_TOKEN}&fields=creator_uid
Using the app token you will then get the email address when looking up the creator_uid in the graph api:
https://graph.facebook.com/{creator_uid}?access_token={APP_ACCESS_TOKEN}