Adding HTML just to a block
I just tried this module today when I saw your post, if you don't want the 'read more' to appear on a full content node display you could create a new display template and add the php path/read more html link on the template you created then choose it from the render mode of node in block settings.

But I'm a little curious, why did you not use views block instead? it's a lot easier, you could just put the read more link on the footer.