I came across this question myself and there is no possibility to get an spsitedataquery from client object model.
What I did was writing a short webservice myself which did an spsitedataquery and queried this webservice from client side.
Reference: https://msdn.microsoft.com/en-us/library/ff798388.aspx
And this question has already been asked: Client Object Model Equivalent of SPSiteDataQuery?