As belisarius said, there is no current support for hatching but you could use a graphics overlay as a workaround.
First create the desired pattern and texture a Polygon
with it:
t = Table[{0, n}, {n, -1, 1, 0.1}];
g[c_] := Graphics[{AbsoluteThickness[8], Line /@ Transpose[{t, t + 1}]},
PlotRange -> {{0, 1}, {0, 1}}, Background -> c];
pattern2[p_, c_] := Graphics[{Texture[g[c]], Polygon[{
{p[[1]] - 1, p[[2]] - 1},
{p[[1]], p[[2]] - 1},
{p[[1]], p[[2]]},
{p[[1]] - 1, p[[2]]}},
VertexTextureCoordinates -> {{0, 0}, {0, 1}, {1, 1}, {1, 0}}
]}]
pattern2[{1, 1}, Purple]

Create a function to place the pattern over any field of a given value n
. matrixLength
is the length of the input data, c
the color.
overlay[patternFunc_, n_, c_, matrixLength_] :=
Show[patternFunc[#,c] & /@ ({#2,matrixLength +
1 - #1} & @@@ Position[data, n])];
Example:
plot = MatrixPlot[data, ColorRules -> {1 -> Red, 2 -> Orange, 3 -> Purple}]

Show[
plot,
overlay[pattern2, 3, Purple, Length@data]
]

Not the most elegant/efficient solution but it might be useful as a starting point.
Purple
above inColorRules
withGraphics[{Black, Line[{{0, 0}, {1, 1}}]}]
(I can fudge with the color later; the hatching is most important to me). But with that, I simply get this weird maroon color with no hatching. And even if I switch outBlack
to, say,Yellow
in the above, there is no effect. $\endgroup$ – user32416 Oct 29 '15 at 19:08