Your best option is to come up with a custom solution, as the OnClientMouseOver is a client-side event that is raised, when the mouse is within the rotator's container, not the various items. Here is a sample OnClientLoad handler that attached the mouseover event to each of the items and the mouseover handler sets corresponding current item index:
items = rotator.get_items();
Keep in mind that this solution will result in constant change of the current item. As the items are changed, the mouseover is called for the new item, that is under the mouse cursor, thus creating a perpetual change in the rotator. Therefore I do not advise you to use this solution, as it is not user-friendly. A better option will be to use the OnClientItemClicked event to change the current rotator's item.
Hope this helps. Please, let me know if you run into troubles.
All the best,
the Telerik team