Just a suggestion...go to the pro player forum, start a thread complimentary or derogatory about Federer or Djokovic and you should have all the Serbian members posting within minutes...then ask them if they want to hit!
I'm actually surprised you haven't gotten any responses. I have played in Italy and France but never been to Serbia (but would love to).
I have played in Barcelona (arranged hit here on TW forum) and Italy and Austria.
But I did approach Djokovic Academy in Belgrade and they could arrange private session with one of their coaches for what is a very very low price ... at least comparing to what one would pay here in the US.
Going to buy some Federer gear now for my upcoming trip to Belgrade - wish me luck !