I have already answered your support ticket. For convenience I pasted it here:
"For the time being passing additional parameters to the web service is not allowed but this is already in our ToDo list. We will do our best to implement it for one of the next releases of the control (we will not able to do that for the upcoming Q3 2010 resale, scheduled for the middle of this month)."
All the best,
the Telerik team
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