I find that to be excessively creative given the facts. Had the buildup to civil war occurred in only the decade prior, or if the North postured aggressively and directly threatened the South, or if there was a significant body of historical evidence suggesting this case, it would be worth looking into. As it stands, however, we possess an incredible amount of primary sources which conclusively prove slavery had already been a source of political and ideological conflict among the states for a century. There was legitimate concern shared among the states about just what would happen economically in the South were slavery ousted, because Southern productivity was dependent upon the use of free, forced labor, while the Northern states were urbanizing, industrializing, and mechanizing their means of production.

The largest counterpoint to this suggestion is in the primary sources themselves. If the claim were true, we should see it throughout the secessionist documents we have available. And we do not. Slavery is the primary issue prevalent in all secessionist sources that convinced the states to withdraw from the union. There was mention of worry about the Republican call for tariffs in the 1860 election, but this did not spread across the whole of the South, nor was it a rallying cry among the secessionists as they whipped up public opinion in support and defense of first the secession, and then the Confederacy itself.

Ultimately, the sources speak for themselves. Take a look at the literature of the time, the pamphlets, the various arguments made for the purposes behind and need for secession and war. Wherever the states seceded, and even within those states wherever pro-Union--by which I do not and the people of the time did not mean the North, but the constitutionally declared perpetual union of the States--had an almost singularly consistent theme: slavery. There were parties and factions, candidates and referendums, that self-identified as slaveowners vs non-slaveowners. There were no demonstrable parties or groups or calls for rallying against the North and dividing the Union to repel Northern economic subjugation.

[EDIT]
I worry that I may have failed at directly answering the question by my chosen vector of interrogating secessionist documents, as opposed to those of the Union instead. On that front, to my knowledge, there is ample evidence to suggest that some in the North preferred to allow secession to stand than fight a war to preserve the Union. Some of this has often read to me as the kind of if-you-don't-like-it-then-good-riddance kind of attitude that occurs in these kinds of situations. I don't at the moment have much at my fingertips to state emphatically whether there is significant evidence to support the idea that the North used slavery as a cover to subjugate the South economically. Popularly speaking, this would be difficult to prove. We would need some very strong and persuasive documents from monied interests in the North with significant political power and/or connections, a direct line to the Congress and President, and demonstrated influence on Lincoln's decision to reject secession.

In the end, while secessionist and Northern declarations may have included a varying number of otherwise important reasons for the conflict, there was only one issue that so polarized the nation that one side was willing to break away from the Union and the other was willing to dispute the legitimacy of such an act.

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Interestingly enough, this economic argument actually began immediately after war broke out (which, curiously for those who wish to dub the war one of Northern aggression, was initiated by the Southern states). More curious, however, is that this argument did not have primacy in America. Instead, it was found in the pages of the London press, arguing from afar that the war was one of tariffs and economic issues perpetrated against the "free trade economy" of the South by the North, had no basis in any principles beyond that of the North's lust for sovereignty, and advocating that the North ought to recognize the seceded states. And then, though certainly never taught to any American student who studies the Civil War, a (now) infamous man who was ever watchful of economic matters, particularly of matters that focused on the abuses of economic power for another group's gain, came out to dismantle these arguments in their entirety. His name? Karl Marx.

Marx wrote a series of articles in 1861-62 attacking the London press and politicians for fabricating reasons to explain the war that were opposed to the facts of what was actually happening in the states. In one article, he even runs down a list of seceded states and data that was available at the time showing that border states, like Tennessee, rejected secession, but were then subjugated by force of arms from the Confederacy, who laid claim to such territory as being rightfully "theirs" and places where the institution of slavery ought to exist.

* If you're interested in more of Marx's articles against the London press, these two are the earliest:

http://www.marxists.org/archive/marx/works/1861/10/25.htm

http://www.marxists.org/archive/marx/works/1861/11/07.htm

A complete listing of his Civil War articles can be found at http://www.marxists.org/archive/marx/works/1861/us-civil-war...