Looks like you are on the right track and you just need to get the correct equation for the height of the pyramid.
One thing I've found helpful in similar situations is to sometimes define the size of the grid larger than needed and then use NaN to indicate points that I don't want to be drawn. This helps eliminate the need to conform to a square boundary.
Here's a pyramid example
>> x = linspace(-8,8,200);
>> y = linspace(-8,8,200);
>> [X,Y] = meshgrid(x,y);
>> Z = (4-abs(X)) + (4-abs(Y));
>> Z(Z < 0) = NaN;
>> surf(X,Y,Z)
Hope this helps!
