It has been a long while since this was posted, but as QuickLook still doesn't show previews of HEVC streams in MP4 containers by default, I will add my 2 cents for the Googlers who will end up here trying to find a solution.
Apple operating systems expect the "hvc1" tag to properly recognise HEVC streams in MP4 containers.
Once this tag is added, macOS will properly display QuickLook previews, iTunes will properly recognise the video for the purpose of syncing with devices, and supporting devices like iPhones will recognise the video for playback from Gallery.
Example using ffmpeg:
# -tag:v hvc1 enables support by most Apple devices
ffmpeg -i input.mp4 -c:v hevc_videotoolbox -q:v 65 -tag:v hvc1 -c:a copy output.mp4
# note: if source is already in h265 there is no need to re-encode, in that case just use -c:v copy