maybe a little late, but I ran into the same problem and found a workaround.
You can use the server-side RadScheduler.OnTimeslotCreate Event:
ProtectedSub RadScheduler1_TimeSlotCreated(ByVal sender AsObject, ByVal e As Telerik.Web.UI.TimeSlotCreatedEventArgs) Handles RadScheduler1.TimeSlotCreated
After this you can use 'onTimeslotMouseOver' as callback function in your client-script:
function onTimeslotMouseOver(sender, startTime)
{
alert(startTime);
}
You only need to implement some code for your custom tooltip, e.g. a RadToolTip Control or simply add a <div>-Element at mouse position or something like that.
BUT: This creates a MASSIVE overhead in the Markup-Code (almost doubled the size of my aspx-Page from 160Kb to 277Kb)
But as I use this in an intranet application it doesn't matter for me.