The WWDC 2014 session 'Direct Access to Video Encoding and Decoding' covers H.264 encoding using kCMVideoCodecType_H264, but looking at the CMFormatDescription docs, there doesn't appear to be an equivalent for H.265.
Is it possible to encode to H.265 in a hardware-accelerated manner using AVFoundation?
Updating this question to give an answer now it's a few years later and things have moved forward.
CMFormatDescription can now be used with constant kCMVideoCodecType_HEVC, available since iOS 9.0.
Apple did a session at WWDC 2017 entitled 'Working with HEIF and HEVC' https://developer.apple.com/videos/play/wwdc2017/511/