No, it's not currently possible, and not likely to make it onto our roadmap.
In fact, the whole use of icons for objects to a large extent reflects the history of the Naked Objects idea (going back long before the current MVC implementation). It was a central principle that all objects should have an associated icon to
help the user internalise the object model. Also, the original Naked Objects UI relied heavily on drag and drop and the icon gave you the thing to drag (in MVC drag and drop is currently limited to dragging from the history into a field).
The icon was also what you clicked on to get the actions (in MVC there's an explicit Actions menu).
It was also one my contentions that icons should represent nouns and not verbs (going back to the early work at PARC on GUIs). I think I covered this in my thesis - should you be interested.
If you want to add icons to represent actions then you will need to customise the framework. e.g. you could write your own Html helper methods to overload the standard and write a mechanism to look up an icon for the action.