This addon resolves the featured image issue, along with other shortcomings – e.g., it adds analytics, featured posts and even a menu: https://wordpress.org/plugins/custom-amp-accelerated-mobile-pages/changelog/
Caveat: I deployed this about an hour ago, so it remains to be seen whether Google will now tell me my AMP pages are full of errors. But so far it’s the best alternative I’ve found. There is also another plugin on the repository that goes even further in terms of layout refinements, but it’s shady as heck (e.g., it wants access to you analytics) so beware.
And of course there’s already an AMP plugin on Codecanyon (with terrible reviews), so the gold rush has begun…
