Could a Precise Growth power be used to grow specific parts of your body more than other parts? (e.g. growing an extra-large liver to counter an Affliction or basically acting as the Elongation effect) If not, is there any RAW way to do this, or how would you house-rule it?
To me, Precise Growth is more likely to let you adjust your height and mass to a specific value, say to pair with Morph so that your guy passes the screening where The Green Giant has his security system check to be sure sure that he's the right size after the embarrassing situation where Joe Quick Change managed to fool his scanners with green face-paint and a set of contact lenses, or to allow you to provide just the right weight to replace the idol while your buddy runs out with it.
To get other effects, use a Power Stunt. I've done that before in games, having my Growth guy just grow his hands to get an Area Attack on his punch. Or, as you say, maybe you can grow your liver to justify an Immunity (Poison) or stunt some of your points into Elongation.