There's nothing stopping you from doing it, but you're actually giving away more than $15,000 by doing so. You'd lose the $15k next season (although he'd still be on the roster) and you wouldn't get it again when you graduated 7. I think you'd have to account for that as giving away more than $15k, but somewhat less than $30k in present value.
On the other hand, you could create a huge super-class by the following method: year 1 - sign an ineligible; year 2 - sign 6 freshmen and 1 sophomore (redshirt the Soph); year 3 - sign and redshirt a Junior. That gets you up to 9 in a single graduating class. If you look hard enough, you could probably find a transfer that was initially ineligible and make the class 10 (even 11 if you are willing to redshirt a senior). Maybe I'll try that one day...