The trick is to get EPF Composer to copy the svg file over during publishing.
Example and template elements allow you to attach files, and those files are copied over during publishing.
So here is what I did:
1. Created a template
2. Attached an svg file. It gets placed in the "resources" subfolder of the templates directory in my plug-in.
3. In another element's description (in this case, a concept page) I referenced
the svg file using a relative path:
E.g. <img src="../templates/resources/example_square.svg" />