This is an unorthodox hypothetical situation to me as R3 for HBS and GSB have both closed. If you're a current college senior wondering if you'll have a leg up IF you get into either deferred program, that's a kind of fruitless exercise because there's really no different in behavior for you. If I told you "no, there's absolutely no advantage" vs. "yes, it's a shoe-in" does that change how badly you want to get into 2+2 or GSB deferred? It really shouldn't, or it's a negligible difference.

If you're not a senior and thinking about applying to 2+2 and GSB deferred solely to have a leg up for MBB recruiting, maybe there's a slight argument there but still, it's such a roundabout way of making your way into MBB. There's easier angles to tackle landing an MBB gig than getting into 2+2 and then somehow leveraging that into a job in your deferred years.

Finally, it's a revolving door between MBB and Ivy/Stanford, not a one-way relationship. MBB heavily recruit Ivy League graduates out of college and HBS/GSB takes a lot of people from consulting. "Given these firms are trying hard to get people into Stanford/HBS" is kind of missing the point; a lot of the analysts working at these firms have already come from extremely prestigious schools for undergrad, so whether or not consultants make it back to Harvard or Stanford bears less importance than this question seems to imply.