You should be able to cast spells in medium/light armor as a Shield Dwarf. I've tried this route out for myself as a Dwarf Wizard, so if that's not working that sounds like a bug.
Fair warning, though: only Shield Dwarves get armour proficiencies, not Gold Dwarves. Neither gets heavy armour proficiency, either, so Plate and the like will still shut off spellcasting (as well as some other nasty sideeffects).