MKDoc's atomic level of information is called a component. There are several types of components: HTML, Text, Photos, Images, RSS, Headlines, Links, and even IMAP based discussion forums.
At the moment any MKDoc page can have any number of different components in any order.
While MKDoc pages are only semi-structured, components can be extremely strict and have as many validation rules as required. So it seems to me that what you want is to create a custom MKDoc component with its own template, validation rules.