Interesting idea, but I’m not sure it would work to China’s advantage when you look at the interplay of all the factors. Let’s compare the conventional “buy-as-you-go” Chinese strategy with the “deferred purchase” one you described.
In the conventional strategy, Chinese reinforcements get fed regularly into the fight, which in principle slows down Japan’s advance in China. This reduces the chances that China will ultimately be completely conquered by Japan, and thus reduces the chances that China will be left with unspent IPCs. Indeed, by spending all of China’s IPCs at every round of play as each batch of new money comes in, the Chinese player ensures that he’ll squeeze the fullest possible use out of his meager resources and thus slow down Japan to the maximum possible extent. As you pointed out, Japan can’t capture China’s unspent IPCs (because the rules don’t allow it), so it has no incentive to conquer all of China from that particular point of view…but it does have an incentive to capture Chinese territories for the purpose of appropriating the new IPCs they generate at each round (and to keep those new IPCs out of Chinese hands).
A deferred purchase strategy would, in principle, increase the chances that Japan will conquer all of China because less resistance is being offered by China to the Japanse advance. Japan gets an immediate benefit (it gains IPC-generating territory) and China is increasingly placed at a disadvantage as it loses ground because the number of IPCs being added to its banked reserves keep getting smaller with each round in which it loses territory. A potentially bigger problem, however, is the flip side of the “pop-up force” situation you mention. It’s true that Japan would be presented with a big headache if a fully-occupied China were liberated by an Allied power, and if this resulted in a large Chinese force suddenly “popping up” somewhere…but the flip side is: what if China doesn’t get liberated? It would mean that all of those banked IPCs would be wasted. Putting them to certain (but mundane) use earlier in the game might be a better investment than hoarding them for dramatic (but non-guaranteed) use in the future.