I like the idea, and I doubt it wouldn't be out of reach for SE to implement. If we can set the level, stats and resistances of these dummies we'd have a much easier time finding our ideal personal setup for what we do best.
Otherwise the next best thing is to gather some friends and go after some higher level monsters you wanna test your damage or spells on. Be sure to go after things that can potentially benefit everyone. Such as gil drops or sea organs/tissues etc.