Added in User Products for WooCommerce plugin version 1.3.1. Took me longer than expected, sorry about that. I did it in more general way, so now you can add any custom meta field, including our EAN. There is now new “Custom Fields Options” section in “WooCommerce > Settings > User Products”. You need to set “Meta key” there to _alg_ean, and set some “Title” for it, for example: EAN.