One reason this wouldn't work:
It would require these big box retail stores to take a pretty substantial loss in order to help out an industry that they're not directly affiliated to.
As mentioned by several other people in the thread, Free Comic Book Day is a way for comic retail stores to draw large crowds and then benefit from them. It requires them to invest a not-insignificant amount of money to bring in product in the hopes that these customers will buy stuff in addition to the free comics. This is something that's nigh impossible to do in a large retail chain, as it probably wouldn't reap the same benefits for a store that doesn't specialize in comic books.
In fact, there's a totally different purchase model you'd have to go by as well. Unlike small retailers which benefit from FCBD as much as the publishers, I'd be hardpressed to find a major retailer who'd think that giving away comics instore would significantly boost their sales. In fact, since it's a promotional thing, I'm willing to bet that the retailers would charge Marvel/DC/Diamond Comics to promote the thing. Since Nac is a saavy person, I'm sure he's aware that distributors/manufacturers pay either a fee or give a lower margin to every item that's promoted in-store. That Coke cooler by the cash register? Paid for by Coke on top of a yearly fee to keep Pepsi from replacing it. That special endcap dedicated to Britney Spears' Greatest Hits? Paid for by the record label. For a nationwide one-day promotional campaign, there'd be a significant fee involved.
Would it be worth it? Probably not, since no one's done it yet. It's not as if these multi-million dollar companies have never thought "Let's put comics in Wal-Mart" before. They probably ran a few cost analyses and decided that it wasn't enough money to be made there. Or even better yet, decided not to play hardball and drop their margins to a near-unprofitable level to get their stuff in Wal-Mart.