Can I use params in Action or Func delegates?

How about this workaround?

private Action<string, object[]> writeToLogCallBack;
public void WriteToLogCallBack(string s, params object[] args)
{
  if(writeToLogCallBack!=null)
    writeToLogCallBack(s,args);
}

Or you could define your own delegate type:

delegate void LogAction(string s, params object[] args);