This is a known limitation of styling slots via handling TimeSlotCrteated and using the modal advanced form. The reasons for this are exactly as you have pointed out.
As a possible workaround, you can set AdvancedForm-Modal
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed