A Unicode hair space character (U+200A) should render as a very narrow space. On the Windows platform (WinXP) Qt renders it as a black bar. On Linux it renders as it should.

Here is an example program:
#include <QApplication>
#include <QLabel>
#include <QFont>
#include <QString>
#include <QChar>

int main(int argc, char* argv[])
{
QApplication app(argc, argv);

QString str("Hello there");
str[5] = QChar(0x200a);
QLabel window(str);
window.setFont( QFont("Sans", 50) );
window.show();

app.exec();
}

There should be a space between "Hello" and "there" in the output Window. This was tested on Windows XP SP3 x86.