Catalysis of the reaction: 1L-1-O-(indol-3-yl)acetyl-myo-inositol + UDP-D-galactose = 5-O-(indol-3-ylacetyl)-myo-inositol D-galactoside + H+ + UDP.
UDP-galactose:(indol-3-yl)acetyl-myo-inositol 5-O-D-galactosyltransferase activity