Maybe you want to set matrix_element_mul to "."

(%i6) matrix_element_mul : "."$

(%i7) a: matrix([1,2],

[3,4]);

b: matrix([a,a],[a,a]);

c:b.b;

(%o7) matrix([1,2],[3,4])

(%o8) matrix([matrix([1,2],[3,4]),matrix([1,2],[3,4])],[matrix([1,2],[3,4]),matrix([1,2],[3,4])])

(%o9) matrix([matrix([2,8],[18,32]),matrix([2,8],[18,32])],[matrix([2,8],[18,32]),matrix([2,8],[18,32])])