I've not tested, but something along the lines of the following
should work:
Code:
QTextBlockFormat bf = this->textCursor().blockFormat();
bf.setLineHeight(lineSpacing, QTextBlockFormat::LineDistanceHeight) ;
this->textCursor().setBlockFormat(bf);
I wouldn't think adding a preference with very limited options would prove terribly difficult. Might need to check there's no performance hit, though.