Hi Evan,
Based on my research, official solutions for letting federated users sign in to Office 365 when ADFS is down have not been published presently. Breaking the federation relationship manually when ADFS is down may cause the issue that the federated users cannot be authenticated via SSO after the ADFS service comes back.
When you deploy SSO, please consider the high availability of the network infrastructure and the ADFS service, such as deploying an ADFS server farm, is recommended. Actually, We recommend that you deploy at least two federation servers in order to provide fault tolerance, load-balancing, and scalability to your organization's AD FS production environment.
Form more detailed information, please refer to the article below:
http://technet.microsoft.com/en-us/library/jj151794.aspx#bk_deployfsfarm
Thanks,
Aaron Yuan
