Unfortunately, that's not available as of now. In fact this is the first such request we are having. The show more results arrow (down caret) is rendered as is with no logic applied and it can not be overridden.
I completely understand your point -- to use font for icons and it's a valid point.
Yet without any modifications, just from the API attributes that can not be done.
We are almost at the 2013 Q1 release, so such addition will be impossible, yet we could consider it for a future update.
the Telerik team
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