How can I make Craft's caching aware of fields used inside a plugin?
The {% cache %} tag tracks Elements (Users, Entries, Categories, Tags, etc.), so if you're want to do this using a plugin, then the plugin's field type needs to inherit from BaseElementFieldType and needs to implement its own element type.

From there, it's just a matter of calling craft()-&gt;templateCache-&gt;includeElementInTemplateCaches().