Thank you @Magic3 . A very sensible and considered approach. I hope you are successful with this.
I host long-term guests, so apart from the guests who left months before their stay was due to finish (not entitled to full refunds and not asking for them, but given them by Airbnb anyway), I have four long-term guests currently booked in the coming months. None of these fall under the extenuating circumstances. I already contacted the next two guests to ask about their plans well before the new policy and they were unsure at that time but have since decided to cancel.
Yet, no cancellations have been made. One guest is being lovely about it and keeps saying she is going to cancel, but doesn't. Although she has told me she thinks it fair that she takes a partial refund (with the promise of the rest should I be able to rebook the room), Airbnb keeps prompting her to request a full refund. The other is refusing to cancel unless she gets a full refund and is sitting it out waiting for Airbnb to extend the dates of the EC policy, because this is what they told her to do. Bear in mind these bookings block my rooms for months, not days, at a time.
In light of all these issues and Airbnb's tactics to get guests not covered by EC to demand full refunds, I decided not to contact the next two guests who are due to arrive in the Summer. I would like to employ your strategy, but don't want to open up another can of worms. I don't trust Airbnb at all to handle these cancellations in a fair and ethical manner. Even when the guests tell them they WANT to pay me something, Airbnb convinces them not to.
Of course, Airbnb will probably extend the EC dates and those guests will then be covered. So, trying to buy time might not be the best solution.
I have decided to advertise the rooms elsewhere and see how it goes. I was waiting for the cancellations, but I can't wait anymore. If I manage to rent the rooms elsewhere, I will refund the guests anyway for whatever part of their stay is covered.