If you're not going to mirror the content at both locations then a round-robin solution will not work, as BIND has no concept of "down" and will dole out both IPs all the time. The best you can do is run a polling script that tests for an appropriate response time from the server and then dynamically modifies the zone file via nsupdate, but this would also mean moving your DNS service (and poller) to a third machine somewhere else, and keeping a very low TTL value.
To get back to my original question, I wonder if there are any third-party DNS providers who would do something like that. Like those managed DNS providers (although the ones I have seen are more basic than WHM). Perhaps only corporate providers offer that functionality.