Thank you for the proposed new feature - I forwarded it to our developers for consideration.
In the meantime, you can use clientside api
of the RadToolTip control to achieve the desired result. For example - add all target controls to the TargetControls collection of the manager but do not use the AjaxUpdate event. Instead, handle the ClientBeforeShow/ClientBeforeHide events to call the WebService and then set the HTML that the WebService returned as content of the RadToolTip by using set_ContentElement method.
the Telerik team