The advantages of creating your own wysiwyg editor is that you can control exactly what functionality Jasonette supports and allow developers to create views across different platforms (right now Xcode is Mac only). But have you considered starting with Interface Builder and creating a tool to convert IB views to JSON? Interface Builder is home to a lot of developers who already understand how to use that tool, asking devs to work in a new half baked editor would probably be frustrating and add more complexity/moving parts which is your enemy as a new project/endeavor.

If you create a tool that just converts IB views to JSON you can demonstrate progress much faster and learn and experiment with what the requirements of the final editor are. It will be much easier to stand up and way less effort than maintaining your own editor codebase while the project is young. Your time is the most valuable thing you have so don't spend it creating a new editor which could be its own project in itself. You want to put it towards expanding missing functionality between JSON and native components. One day when you find success and if you really still need to limit the user you can move all the functions you made for converting Interface Builder views into your own custom editor.

I think first thing I would do is take a very complicated interface view from a native project and analyze the view hierarchy and report errors to the user like "Warning: Nested UITableViews are not supported in Jasonette", "UIButton delegate will be ignored", etc. Then once you create this sort of 'unit test' that the IB is well-formatted you can then go about exporting it to JSON.

There may be a world where you actually are able to use all of the functionality of IB and don't ever have to have your own editor.