You only need the outage to last for the window of [begin renewal attempts, expiration], not the entire 6d lifetime.
For example, with the 90d certs, I think cert-manager defaults to renewal at 30d out. Let's assume the same grace, of ~33% of the total life, for the 6d certs: that means renew at 2d out. So if an outage persisted for 2d, those certs would be at risk of expiring.
https://letsencrypt.org/docs/rate-limits/
For the “exact same set of hostnames” (aka. renewals) the rate limit is 5 certificates every 7 days.
So you could do it every other day, if you can make sure there's only one client doing it.
And they're very clear this is a global limit: creating multiple accounts doesn't subvert it.
So you'll need to manage this centrally, if you have multiple hosts sharing a hostname.
https://community.letsencrypt.org/t/post-to-new-order-url-fa...
If they do start providing 6-day certs I hope their turnaround on issue reports is faster than that (and ideally have something better for reporting issues than a community forum where you have to suffer clueless morons spamming your thread).