OK, so I decided to answer this question. At the moment we have two big bond issuances outstanding, one for $88,000,000 from January 2011, and another for $126,000,000 from February 2012. (Both of these include portions refinancing older debt, including the GOL-era debt expanding the north stands. Remember those days of attendance optimism!) I looked up the original offering statement for the Feb 2012 bonds.
Here's a link for the curious. It quite plainly states that
So there's no security of any kind for these bonds. And the statement it is also quite clear that no other organization, like GT itself, or the Foundation, or Dave Braine, is on the hook for these debts. That said, obviously there are all kinds of institutional reasons by GT would step in. But it's not legally required to, at least not by the bonds.