Ahem, yes. You'd think I might have noticed that very conspicuous bolded word
Determinant 
So, the best I can do is a really ugly kludge that produces
.
Code:
\begin{array}
{r|@{\hspace{9pt}}c@{\hspace{12pt}}c@{\hspace{9pt}}|}
\multicolumn{3}{r@{\hspace{1pt}}}{\overbrace{}^{\textstyle m}
\overbrace{}^{\textstyle n}} \\ p\,\{&A&B\\ q\,\{&C&D \end{array}
This has the drawback that it isn't well aligned vertically. If you want to put it in a formula like
then you'll want to raise it by putting it into an \hbox along with a \raise command, as in
.