I see. Probably the easiest way to replace the built-in tooltip is to first disable it by setting RadScheduleView.TooltipTemplate=null, then create custom appointment template and set a 3rd party tooltip inside, such as this one that is much more powerful than the standard:
As far as I know Silverlight 5 will not enhance the tooltips, but you should be able to simplify the above suggestion just by binding the custom tooltip's attached property in the appointment style (instead of creating custom appointment template).
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