Well, gtk-qt-engine, which this bug is about, does precisely that. The trouble is that it doesn't work very well, especially for Firefox.
I don't agree that QtCurve is a hacky solution: it merely provides a GTK theme that doesn't look out of place among Qt applications. I can't see most people worrying that not every app looks exactly the same. And if they do, they can use kde-style-qtcurve as well.
Of course, you could always use the qgtkstyle you mention and just let GTK do all the actual rendering!
Well, gtk-qt-engine, which this bug is about, does precisely that. The trouble is that it doesn't work very well, especially for Firefox.
I don't agree that QtCurve is a hacky solution: it merely provides a GTK theme that doesn't look out of place among Qt applications. I can't see most people worrying that not every app looks exactly the same. And if they do, they can use kde-style-qtcurve as well.
Of course, you could always use the qgtkstyle you mention and just let GTK do all the actual rendering!