Given the same position vector that would be used for rectangle():
% the position parameter used by rectangle()
pos = [0 0 20 10]; % [x y w h]
% rearrange to create a point list
x = pos(1) + pos(3)*[0 1 1 0 0];
y = pos(2) + pos(4)*[0 0 1 1 0];
% plot it
plot(x,y)
If you intend to replicate other functionality of rectangle(), that will be more complicated.
