[theorique]

Furthermore, assuming that one's 'consciousness' could actually be brought back in some kind of form that roughly approximates what it was like when one was alive, what's to prevent one's body/brain from being used in some kind of terrible medical experiment?

For example, what if an evil scientist or organization were to buy up cryogenically preserved bodies on the cheap in 300 years?

Yeah, you could say it's unlikely, but why is it more or less likely than being unfrozen into some paradise?