One option is to provide dedicated support while letting the code be open. There are plenty of businesses that run on open, freely available architecture, and they make all their money by selling a "Pro" plan that allows people the security of being able to pick up the phone and ask a question. However, you have to give away your product on the front end as a loss leader. This is essentially the "Insurance" business model. It's how Circuit City rose and fell. Be careful, and good luck.
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.
I also build Wordpress plugins, so this is a great question for me to answer.
I think building an API to add open source code like PHP and HTML 5 would be great. Using a Restful API is the best way to go so web designers can simply send their data to the directEdit plugin and it will dynamically create the website.
I would be glad to speak with you on ideas for offering an API.
I always look for open source APIs to allow me to modify the data