I also think the tooltip is useful in your case. However, in Silverlight disabled elements do not show its tooltips by design.
You could workaround the issue by disabling the contents of the tab item (header and content) and restricting the selection. The point is to leave the item enabled and disable the contents except for some transparent element holding the tooltip.
Another approach would be to manually place the tooltip over the disabled tab item.
All the best,
the Telerik team
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