How to implement Selective Refresh within posts?

Yes, it is possible. In fact, look at the Customize Posts plugin where selective refresh of partials for title, content, excerpt, featured image, etc is implemented. You’ll note that it depends on the settings for posts being registered dynamically along with the partials for each post. The setting ID must be variable and contain a reference to the post ID that it is associated with. For example, in Customize Posts a setting containing the object for a given page may have an ID that looks like post[page][123]. Then it will have partials dynamically created like post[page][123]How to implement Selective Refresh within posts?, post[page][123][content], and so on. I suggest building on top of the foundation in Customize Posts. You can refer to this examples issue for some direction on how to add support for custom postmeta.