Isn't that easily solved by rewriting the From header and re-signing the email? Send it from:

Some List on behalf of John Sender <list@example.com>

... instead of ...

John Sender <john@other-example.com>

And now the list software can generate its own (valid) DKIM signature.

EDIT: Nevermind, listen to dbqpdb[0] instead. ARC sounds like a better way to go.

[0] https://news.ycombinator.com/item?id=21420732#21422328

But you are right. This is what lists have been doing. They may also set the X-Original-From or X-Original-Sender headers. Google Groups does it anyway. ARC will be better than that though, of course.