Modify the class info.magnolia.cms.taglibs.SetNode$NodeMapWrapper to handle EL expressions such as "${content.childContent}" so that usages such as...

<cms:setNode var="content" />
<cms:setNode var="myContent" contentNode="${content.myChildContent}">

...can set a child node to a variable. Currently, only "built-in" properties (such as title) and child NodeData objects are supported. This would also be helpful in other tags that are based on the BaseContentTag (i.e., any tag that can has a "contentNode" attribute).

It should be easy to implement: in addition to handling child NodeData nodes when calling "containsKey" and "get", handle child Content nodes as well. Of course, the "size()" method currently returns the size of the NodeDataCollection, so I don't know how to handle that.