The QFontDialog supports overline, underline and strikethrough. It has checkboxes to enable strikethrough and underline, but no checkbox to enable overline. This is inconsistent with what is supported and should be added.
It's possible to work around this by hacking QFontDialog or creating a subclass and altering elements through the child objects contained in the dialog but this is messy and not very portable.

The attachment shows a messy work-around but it would be trivial to add the additional checkbox back into QFontDialog's private class.