Get message body for collection of new messages?

I believe that all you have to do is add the following suffix to the REST API call:

restapi.format_detail=full_list_element

So in your example it would be:

http://community.lithium.com/community-name/restapi/vc/boards/id/video_cards/messages/linear?page_size=100&page=1&restapi.format_detail=full_list_element