Well, there is an option to have error pages on your heroku domains, but that is probably dependent on at least a minimum level of the routing layer working.
Yeah, I have that setup but its not being used at the moment with this outage - if full redundancy with multiple regions is a huge challenge, perhaps some interim basic routing redundancy would go a long way so we could at least display a branded error page during an outage.
https://devcenter.heroku.com/articles/error-pages