How to render / preprocess list of entity reference nodes? I hove a content type that editors can 'attach' related nodes to. 

When I 'theme' / 'style' nodes of this content type through view mode everything is basically fine: the related nodes are rendered as teasers and I can style them as I like.

Now I need to wrap the list of related nodes in an extra class e.g. 'related-nodes' according to the output of another field. For example the editor chooses 'round corners' from a list field, then the css class "rounded" should be added to the wrapper of the list of related nodes should. 

I am aware I can remove the entity reference field from the display settings and preprocess it through template.php. But then how do I preprocess the list in template.php? So far I just don't know how to actually access / handle it.

In devel tab I do get the array of related items. I could of course loop through that array and re-build node by node thruogh node_load – but since I only want to wrap that loop re-rendering each node feels wrong.  Any suggestions?
As per @Clive comment, Fences module may be able to help with that.

A quite straightforward option would be to implement hook_node_view() in a custom module and add a #prefix and #suffix to the field element (prefix of &lt;div class="foo"&gt;, suffix of &lt;/div&gt;)