i don't know much about 1.5" stuff but i think what you need is a shim with an O.D. of 38.1mm (1.5") and an I.D. of 28.6mm (1 1/8"). those reducer cups should have an O.D. of 49.7mm which is the cup skirt diameter of 1.5" head sets an an I.D. of 34.0mm (1 1/8" cup skirt diameter). thus they won't work even if long enough and modified. that's 5mm wall thickness for the shim - not sure if it would be flexible enough to warrant sufficient clamping force (if that's a word).
1 1/8 ahead stems are plenty and cheap - is it really worth the hassle and in the end still having a half decent set-up?
Carsten