I don't know the answer to the JSON submit question. The underlying Ajax class does support submitting a jsonData object, but I don't know if the Submit action will support that down the line.
Custom component support will be available in a near future release of the Designer. It will involve programming a "contract" or "plugin" to the Designer, which will basically describe your component's attributes and behavior to the Designer, and put the component into the Toolbox for you to use. This will not be in the next release but will be coming soon enough. In the meantime there is no support for this, sorry.