Yes, Windows Server 2012 R2 is support as of Office Web Apps 2013 SP1
The Planning page TechNet article has been updated to include Windows Server 2012 R2 in the supported OS list. http://technet.microsoft.com/en-us/library/jj219435. There is an important Windows .NET Framework update you need to install for things to run more smoothly, however. Read on…
Problem with Office Web Apps viewer testing
With the continued adoption of Windows Server 2012 R2, more issues with Office Web Apps 2013 are surfacing. One glaring item from a testing standpoint is the inability to use the Office Web Apps test viewer to view Office documents.
No matter what file type, you will get a general "Sorry the <Office Web App> ran into a problem.." message whenever the view.aspx page is invoked by Office Web Apps. Thankfully, this issue does not affect normal Lync, Exchange or SharePoint integration, but it does make testing nigh impossible.
Answer: .NET Framework 4.5.2
As a general practice, it is good to keep servers up with .NET Framework updates. The 4.5.2 update for .NET Framework resolves the above issue with Windows Server 2012 R2, and may resolve a number of other as-yet-to-be-discovered issues with Office Web Apps. Currently, using the Office Web Apps test viewer is the only confirmed issue on Windows 2012 R2, but other unconfirmed issues may also be corrected by applying .Net Framework 4.5.2. If you encounter any Office Web Apps related issue you suspect may be specific to Windows Server 2012 R2, please make one of your first actions to install the .NET Framework 4.5.2. Remember applying the update will likely require a reboot.
Here is the link: http://www.microsoft.com/en-us/download/details.aspx?id=42643