Not from experience, as my researcher days ended before open access really took off, but if you can't convince them based on the ethical advantages, I would approach it using the "open access citation advantage carrot".
Impact factors are journal-wide, but there's a number of studies that have looked at the effect open access has on the number of citations of individual papers, which is a very important metric for just about every author, and the consensus is that open access works are in fact cited more: http://sparceurope.org/oaca/
Same goes for open data and the open data citation advantage: https://peerj.com/articles/175/
I'm sure someone with real experience in convincing co-authors will come along and support or criticize this method. Please report back with your experience as well, it will be useful for a lot of people.
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