I have a few datacenters within my company. In one DC, we own a BGP AS and the pubic IPs. That DC gets internet locally and services customers in the local area. In another DC, we have our own BGP AS as well, but we lease IPs from two providers. We have failover working between those two providers in a semi-load balancing way. i.e. we adveritise our leased IPs to both providers. What we have discoverd is that both providers take the same fiber out of the building so we are at a single point of failover. Between our DCs is a MPLS Cloud with AT&T that also connects about 10 other offices.
What we would like to do is set up a way that we start to advertise our AS out of the first DC when the second DCs fiber gets cut. How would we set up the first DC to become the "default-information originate" when the internet at hte second DC goes down? This would swing the internet traffic out of the first DC. The second part of the question is how would we begin to advertize our routes out the first DC? The really fun part of this is we need things in our DMZ off of the FW in DC2 to go over the MPLS network and re-natted out a FW in the first DC. I can easly buy another ASA for the first DC if needed.
It's possible you need to make a eBGP peering with DC1 on DC2. And if in DC2 there are more than one BGP running router you can use LOCAL_PREF attribute to make default route from ISP more (higher is better) preferred than from DC1 and if there is only one router use WEIGHT attribute (higher is better).
About DMZ if you are redistributing default route learnt from ISP to IGP it should be ok. Just you need put routes back to source (to DC2) and NAT configuration to permit those subnets on firewall or router that is doing NATing.
Hope it will help.
If I correct understand your network topology, you can use conditional advertisement for injecting BGP route toward your second DC ISP.
What IGP do you use? If you use OSPF, you can forcing your IGP to disable "default-information originate" also using conditional advertisement, if you specify route-map.