There seem to be some fonts that include fractions with horizontal fraction bar (like this: Fraction Fonts by Jim Bennett - I have not tested it). Or you can create the whole expression in a program with math equation capabilities (like Word) and paste it into Indesign.
paste as what? some anchored object? that what i've done
i don't understand why i cannot rule the vertical positioning within object style. Probably i can fix it while changing a code but in a book with hundreds of such fractions it seems timeless.