After further investigation, it looks like the following is not even possible:
> Patching connectedCallback and attributeChangedCallback directly on the node feels wrong and it's a very dirty approach.
Not only those two callbacks are triggered indirectly with the node as context, even monkey-patching the Class, after it's registered, won't make the upgrade observable when a Custom Elements goes from a template to a live content.
In few words there's no work-around if not through timers/rAF/thenable checks.
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