Default featured image for products

Starting a new thread after cstevenson noted this https://premium.wpmudev.org/forums/topic/need-some-help-styling-my-marketplace#post-136076

Hopefully he'll chime in with some advice here too.

I need way to automatically assign a default featured image across the network, but only to marketpress products. I have a few lazy sellers who haven't added featured images yet and it makes the marketplace look bad.

Advice and guidance needed.

Thanks!