You would need to remake character model's UV maps and then remake all other items to fit those UV maps.So not possible at all. You simply can't do this in WoW.
It definitely is possible and it would work, but the question is how to make such model fit different races and genders. You may need to make multiple models for different characters. How to equip a model on chest? By using spells. Check spell 45450 (and its visual effects) to figure out how to make this. Its actually fairly simple, the only challenge is to make that model.If you want this for multiple character models, you will quite likely need to make multiple items with multiple spells with multiple chest models. Inconvenient, but doable. By using some server side script you would be able to make just one item which would choose correct spell for different player's race and gender. What you probably won¨t be able to do are multiple texture variations. Models of such items will need to have hardcoded textures, so in order to make multiple items with different textures, but the same models, you will quite likely need to make multiple M2s (and spells for them and so on... bleeeh). Tedious, but again, still doable.