With the 3.6 update, you can now build Responsive Layouts directly in InDesign and export with in5.

Using the Ajar in5 Embed WP plugin, set a responsive Width (e.g., 100%) and check the Responsive Height box. 

When you check the Responsive Height box, the Height setting is used as a default and a minimum. As the Width of the frame is scaled, the Height will be adjusted to match the layout. 

You can see it in action in the video below.