Mark, that's right. render_block is called, but render_item and render_feed are not. My thinking was that it might be possible to extend block_rss_client itself and not have to extend the renderer.
I was trying to provide maximum flexibility without a good understanding of how the functionality would be overridden/extended.When exporting the block for templating, the feeds and items are too. Then they are rendered with an include inside the mustache file.