Is it possible to put the "gliss." text below the glissando without hacking?

I think not.
It might be good to have this as a style option (above/below) and also respond to the Flip command (X). Feel free to add it as a S5-suggestion into the issue tracker: https://musescore.org/en/project/issues/musescore

Thanks for #313490: Place glissando text below