Such diagonals are definitely possible using graphic packages like tikz
/pgf
or pstricks
. However, here's one using TeX leaders:

\documentclass{article}
\usepackage{amsmath,graphicx}
\newcommand{\diagdots}[3][-25]{%
\rotatebox{#1}{\makebox[0pt]{\makebox[#2]{\xleaders\hbox{$\cdot$\hskip#3}\hfill\kern0pt}}}%
}
\begin{document}
\[
\begin{bmatrix}
0 & -2 & 1 & & -1 & 2 \\
2 & & & & & -1 \\
-1 & & & & & \\
& & \multicolumn{2}{c}{\smash{\raisebox{.5\normalbaselineskip}{\diagdots{8em}{.5em}}}} & & 1 \\
1 & & & & & -2 \\
-2 & 1 & \phantom{-2} & -1 & 2 & 0
\end{bmatrix}
\]
\end{document}
The minimal example provides \diagdots[<angle>]{<len>}{<skip>}
that draws a diagonal array of dots (actually \cdot
s) of length <len>
at an angle of <angle>
(default is -25
). The <skip>
defines the approximate length between dots.
I've placed \diagdots
in the middle of your bmatrix
(horizontally by using \multicolumn{2}{c}{...}
and vertically by using \raisebox{.5\normalbaselineskip}{...}
), and \smash
ed it to remove any vertical height distortion. The \diagdots
output has zero width (by virtue of \makebox[0pt]
).
You can play around with the lengths and angles so see what suits you.
!
in front of it to turn it into a link. A moderator or another user with edit privileges can then reinsert the!
to turn it into an image again.