You could make your own Operator style function:
In[1]:= transTransform = OperatorApplied[#2 . (TranslationTransform[#1] // TransformationMatrix) &, {1, 2}];
In[2]:= vec = {1/8, 1/8, 1/8};
matrix = {{3/4, 5/8, 5/8, 0}, {5/8, 3/4, 5/8, 0}, {5/8, 5/8, 3/4, 0}, {0, 0, 0, 1}};
In[4]:= transTransform[vec][matrix]
Out[4]= {{3/4, 5/8, 5/8, 1/4}, {5/8, 3/4, 5/8, 1/4}, {5/8, 5/8, 3/4, 1/4}, {0, 0, 0, 1}}