Corporations aren't mentioned because they didn't exist yet. A literal reading, or a reading appealing to framer's intent (from either side) is futile, because it tries to put a lot of meaning onto what is effectively an accident of syntax and style that was, at the time, meaningless.
This is not to say that your conclusion must be invalid, just that this line of reasoning is suspect.
Corporations have been in existence in Western culture since at least the high middle ages. Corporations existed in the Americas since before the American Revolution was even an idea. The Hudson's Bay Corporation is the oldest commercial corporation in North America and one of the oldest in the world, incorporated in 1670, over 100 years before the American Revolution and 120 years before the Constitution was ratified and the Bill of Rights ratified. One of the U.S. founding fathers--Benjamin Franklin--was an entrepreneur and businessman, and he used his position as a businessman to engage in all sorts of (often underhanded) political pamphleteering and advocacy.
Corporations most certainly did exist, and without a doubt engaged in political activity. While the doctrine of corporate personhood and the equivalence between money and speech are later inventions, it's not clear that the founders would have supported a reading of the Constitution that allows bans on corporate (or any other) political speech. Especially since it was common practice at the time, and continued to be a practice after the ratification of the Constitution.
This is not to say that your conclusion must be invalid, just that this line of reasoning is suspect.