Given travel operators have to pay to be in ABTA I'm not sure financially struggling (or other) zoos would want to add further to their costs given in the event of zoo failure other zoos already do the best they can to house the animals.
I'd say the chance of "zoo failure" resulting in animals not being found alternate homes in the last thirty or so years is rare.
Basically if zoos/"sanctuaries" have spare capacity they will endeavour to help as far as they can. Beyond that you can't really suggest zoos keep spare capacity "just in case" they needed to help out, costs would be massive.
It could be argued that the zoo should produce a business plan when applying for a zoo licence (maybe they do) but, from experience business plans are no guarantee of preventing failure and, like an ABTA-type membership, may just add a further layer of administrative costs without much tangible benefit.