I’ve have been mulling how to respond here.
Fully is very hard to qualify. There will be multiple paths to monetise Extensions, which are likely to continue to roll out and be enhanced over time. For the initial offering of digital goods purchases in Extensions, developers will be able to retrieve entitlement information in order to be able to fulfil purchases. Per Extension attributable revenue reporting will also be made available to Broadcasters and Developers. Please bear in mind that bits in Extensions is coming too.
I do not think that this is really what you are asking for though! At this time, I have not heard of any conversations around allowing parties other than developers and broadcasters (and Twitch) to track Extension monetisation. This does not mean that it will not happen mind you!
Hopefully this helps, at least a little.