Are placeholders supported in @CHUNK?

As of ContentBlocks v1.9.0, yes they are! :)

You can specify placeholders in the chunk name itself. That's useful if you have subtypes of chunks depending on a field setting. 

For example the following field template:

@CHUNK CallToAction[[+style]]

Assuming "style" is a field setting with value "Card", ContentBlocks will parse that into the following chunk tag:

[[$CallToActionCard?
       &value=`Some Value`
       &layout_id=`5`
       &layout_column=`main`
       &style=`Card`
       ... 
]]