There is no Renderer in the [bd] widget framework for showcase comments. You can't just place template code somewhere and expect it to display data. The data needs to be exposed (which is what a Renderer does for the widget framework).
There is no individual template for that block either (its just template code located in the sidebar template for showcase index).
You can request a new [bd] widget framework render for making comment widgets if you want to (just create a thread in the showcase suggestions). Its probably something that a few people would like to have.
I fully understand what the [bd] widget framework does and how it works. What you are talking about requires DATA be exposed for that to work (which in this case, there is no exposed data). What you want to do can not be done without either creating a renderer OR creating a template with a php callback (which will require programming as there are no methods in showcase setup to use the xen:callback). You'd need to write your own class and methods for being able to use a php callback.