You can define the matrix attribute value as an internal entity in the DTD internal subset.

<!DOCTYPE svg [
    <!ENTITY my_matrix "1 0 0 0 0  0 1 0 0 0  0 0 1 0 0  0 0 0 1 0">
]>

It can then be referenced in the primitive.

<feColorMatrix type="matrix" values="&my_matrix;"/>