The longer answer is, yes this is a supported scenario as long as you use the new Windows Azure Active Directory (WAAD) Module for Windows PowerShell to convert the manage custom domain(s) to federated domains.
This module can be download from the Office 365 portal:
By the way, for those of you who are following my “Configuring an Exchange 2013 Hybrid Deployment and Migrating to Office 365 (Exchange Online)” article series probably noticed that I was planning to used Windows Server 2008 R2 based ADFS servers for the Office 365 federation. However, since Windows Server 2012 are not supported, I have updated part 1, 2 and 3 plus the non-published version to reflect this.
Until later,
Henrik Walther