Hello Francesco,
[...]Would it be possible to have, instead, a contributor agreement that allows contributors to retain copyright while at the same time granting you a non-transferable, non-revokable, exclusive right to relicense their contribution under the ASF2.0 license at a time of your choosing?I feel that this would be a far better solution than copyright assignment...
I retract the original question! I do not intend to persue that path, I am now convinced that instead, I will go for what was proposed. See my reply to Tanguy just now for a proposed text.
Thanks, Kay