[Canvas App/OAuth] Authenticate Salesforce to external app

Hi Community,
I read many guides about the authentication of Canvas Apps to Salesforce, but I haven't found anything for authenticating Salesforce to the external app. I thought to develop Oauth in Apex, and send the access token by a LifeCycleHandler. Unfortunately the two accessible context classes (ApplicationContext and EnvironentContext) don't allow to change the http header. 

At this point I am wondering if it is really possible to do.

Thanks a lot for any idea!
​Michele