Thank you for the question.
There is no dedicated event for the moment when you apply the show fewer buttons functionality to RadPanelBar. I am glad to inform you, however, that such an event will be introduced to the successor of RadPanelBar - RadPageView. This new control will be a part of Q2 2010 due in the beginning of July.
Until you switch to RadPageView, you can use the workaround that you have already found.
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