Interesting concept. I would look at WooCommerce (by WooThemes) for a good open source business model.

WooCommerce, an ecommerce plugin for WordPress, is free and open source. But, WooThemes creates themes and extensions to give more functionality and charge for them.

If directEdit does make designer's life easier, I would look at licensing model to offer it to designers and charge monthly/yearly fee for support and updates.