Do you expect the solution to fail back to the primary DSL when it comes back and to fail over on more than just physical interface up/down? If so things become significantly more complex to the point I'm not sure it's really making life any easier
Honestly, as massive a pain as it might be, manually going through all the sites to make the internal and external network contexts decoupled (I.e. clients not pointing directly at the IP of a single carrier) is going to be a lot more reliable than whatever hacked together solution sounds like a good idea now. Doubly so if you're looking to make that solution itself redundant.
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Do you expect the solution to fail back to the primary DSL when it comes back and to fail over on more than just physical interface up/down? If so things become significantly more complex to the point I'm not sure it's really making life any easier
Honestly, as massive a pain as it might be, manually going through all the sites to make the internal and external network contexts decoupled (I.e. clients not pointing directly at the IP of a single carrier) is going to be a lot more reliable than whatever hacked together solution sounds like a good idea now. Doubly so if you're looking to make that solution itself redundant.