Fences and Field collection

I need a way to add a class to a field collection set.
By this, I mean a single set of field collection fields - not the entire field collection field.
In short, the value of one field within a collection, should set a class on the wrapper element for that field collection set [not the entire field].

It seem the best way to do this is with hook_preprocess_entity, and it's working well.
I get the standard entity markup, with my class in place.

Until fences comes along and blows all those lovely divs away - like it's supposed to.
There is still a wrapping div, but without the class I set.

Any known way to handle this?