Using ElementType as field type for plugin in what way do I provide my own data to the modal?
Ah, in that case you'll need to make your own ElementType. You should probably check out https://github.com/pixelandtonic/Events - that's meant to be a simple ElementType to serve as an entry point while we get some documentation together.

Also, there are some videos to watch over here: http://buildwithcraft.com/docs/plugins/working-with-elements#creating-new-element-types