In reply to Andrew.

The “trick” is to add your Cover block, then within the cover add a Group block and toggle the ‘Inherit default layout’ on, then inside this group add another Group block

One of the reasons I push a bit on the boundaries of what's doable within block themes is to identify and suggest methods for eliminating these "tricks" or "hacks".

It should certainly be easier to accomplish this — and I'm confident it will be over time.