You can control your center of balance with footing, but center of mass is (relatively) fixed. You might affect it slightly by, say, tucking in your legs, but even that would be a slight change.
Fair enough. But isn't the center of mass only fixed as a point within the body? If you duck, the point won't shift in relation to the body, but will in relation to the world around, no? So, I interpret aiming as positioning your body (and, therefore, your center of mass) in relation to the object to be pushed such that it forms a line to the target. The object being pushed is the point that the line is drawn through, much like a pivot. Just make sure that the other end of the line rests on your target and push! I guess I've played too much Zuma and Peggle, eh?
Digression: Would feruchemically tapping pewter change one's center of mass. Which of course leads to the thought...does tapping pewter actually effect one's mass/weight at all, since iron specifically does that? If so, then the two metals have some overlap, and if not, then you have a really light hulk...
First, a nitpick for a nitpick: I believe you meant "affect one's mass" instead of "effect one's mass". While 'effect' can be used as a verb, it is typically used in a sentence as a noun, such as "special effect". When used as a verb, 'effect' means to bring about or make happen, such as to bring about change. The suggested word, 'affect', is typically used as a verb to mean 'influence' or 'modify', which I feel better describes what you were saying. You used it correctly in the first paragraph, so I should probably just chalk this instance up to typo, right?
As to the message, as funny as a featherweight hulk would be, I would have to vote for the overlap position. The Feruchemist could store weight at the same time that they spend strength to get the featherweight hulk effect. I see strength as muscle and, with that, a measure of size. When storing, the Feruchemist is emaciated and weak, shrinking down in size. Muscle mass is decreased. Less muscle, to me, also means less mass/weight. By that same token, more muscle means more mass/weight.
For weight manipulation, I look at it as density control, to a point. As of yet, no character has become intangible from reducing weight, but viewing density control as the method for how weight is manipulated explains why the characters don't change size when storing or spending weight.
Then again, we can just chalk it up to magic, too!
P.S. I love palindromes! How is that pronounced? Sizzle Liz Icks?