I know that question is outdated, but some time ago I was looking for ready-to-use implementation of feature you requested. I digged a bit and implement this for Qt 5 -- take a look at repo.
Main idea is to do:
// Create button what must be placed in tabs row
QToolButton *tb = new QToolButton();
tb->setText("+");
// Add empty, not enabled tab to tabWidget
tabWidget->addTab(new QLabel("Add tabs by pressing "+""), QString());
tabWidget->setTabEnabled(0, false);
// Add tab button to current tab. Button will be enabled, but tab -- not
tabWidget->tabBar()->setTabButton(0, QTabBar::RightSide, tb);
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