Possible solutions.
Option 1. Let the sub frame send a message to top frame using postMessage. And the top frame calls setReportEventDataForAutomaticBeacons() after it receives the message.
Concerns:
A. More complex code structure;
B. We would like to have the navigation ping sent after the top frame receives the message and updates the data. This might not be straightforward to support.
Option 2. The cross-origin restriction is relaxed for such cases.
Concerns: Should be very careful of the privacy issues.

Thanks for the report and the summary!

I'm going to look deeper into how feasible option 2 is. There will most likely be some sort of opt-in system in place, but if I can get the relevant privacy experts on board it might be possible. I'll keep you updated.

option 2 is similar to #152 , in which case, the document created with the FencedFrameConfig opts in with a new response header(probably with the ability to set which cross origin domains are allowed). The cross-origin document opts in by calling reportEvent() with the crossOriginExposed=true parameter.