It is clear that the new IOS 8 action extensions have to be initiated by a user and from a system provided UI. My question is, will the IOS 8 Mail app have a system provided UI so that an action extension can be called when a user is reading an email or writing an email?
Currently, no. I did file a feature request for this, many opportunities for action extensions in Mail! I would suggest that you do the same to possibly get some priority behind this.