I doubt it... you'd be too late for a "normal" refund for choosing not to travel yourself - I know you're saying it wouldn't be your choice, but the point being that's not a way round it.
The Guest Charter states CS will provide compensation for non-availability of booked accommodation. However, the issue you'd have (whilst it's not your personal view) is that a shard berth is deemed an upgrade from the seats (and this is reflected in the respective prices) - so the first row in the table below doesn't apply.
The second row doesn't either - you'd be hard pushed to state a case it was "not possible" for you to travel due to a "fault of the railway" if the seats were out of action and CS offered to take you instead in a functioning berth (albeit shared) at no extra cost.
Likelihood is CS' position would be that you've been offered "upgraded" accommodation (at no extra cost) to get you to your destination - so if you chose to get off the train and not travel, you wouldn't be getting a refund.
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Once the new stock is in, hopefully broken seated coaches will be very rare, but if they did happen, would the "no sharing with stranger" rule to be introduced with the new stock still apply I wonder, or wonder this be overruled in favour of getting everyone from A to B?