The question for the last year has been whether it is supported to customise Exchange 2013 OWA and where the Technet documentation was, if like in previous versions of Exchange it was supported.
Technet Forum threads going back as far as April 2013 and indeed many blog posts exist about toying with the customisation of the OWA logon page, for example, here and here. But apparently no official guidance existed.
So as per the Technet Forums thread that I posted a response to here, I said I would find out where that guidance was from the Exchange PG.
Well that answer came into my inbox yesterday courtesy of Microsoft:
The official Technet Exchange 2013 OWA customisation articles are here:
Customize the Outlook Web App Sign-In, Language Selection, and Error Pages
Create a Theme for Outlook Web App
Now the confusion exists as these are for Exchange 2013, but they inform the reader that they are really for Exchange 2010 SP2.
I have been informed that this will be cleared up and they will correctly say they apply to Exchange 2013 imminently.
So if you want to customise Exchange 2013 OWA within the realms of supportability until your hearts content, then use the two Technet articles above. And if in the meantime you are referencing them and they say they apply to Exchange 2010 SP2, don't be put off, use them confidently.
Oliver Moazzezi - MVP Exchange Server