could you put this in the renderer(s) instead? this is supposed to be a thin wrapper that just injects the renderer, because updating it is painful; everyone using it would need to update their creatives, and we'd need to replicate the same logic in PUC as well. Renderers on the other hand are just shipped together with prebid.

does this need to be set on window?

You can extract this into some shared file within creative/. The 'browserIntervention' event can also be defined once instead of copied as a string, similar to how other events are imported in creative/constants.js (the oddity of that file importing just to export is to avoid pulling more of prebid core into the creative).

window here is not the window where the creative will be; however, from my testing using the "correct" win.ReportingObserver doesn't work - so I guess this is OK.

This made me realize however that this likely won't work when the GAM creative is not set up to use safeframe (which is quite common). The logic there:

doesn't run this renderer at all when the bid provides ad; and runs it using a different window setup otherwise. I'd test this by calling pbjs.renderAd directly (e.g. following https://github.com/prebid/Prebid.js/blob/master/integrationExamples/noadserver/basic_noadserver.html) - we probably need to add the ReportingObserver logic in core as well.