Is it possible to extend Quick Look to support h.265?

H.265 (or HEVC) is the official successor to H.264, with a significant reduction in bitrate for the same picture quality.

However it's not supported widely yet, including in Quicktime, iTunes or Quick Look in the Finder (cmd+space) and I haven't seen anyone talking about support in OSX 10.11 (El Capitan).

I can view files in, for example, VLC, but was wondering if there's any sort of plugin for Quick Look.

QLVideo can display metadata, thumbnails and static QuickLook previews of H.265 video (as long as the H.265 stream is packaged within a suitable container e.g. .mkv, .ts, etc).

Given the architecture of the QuickLook and QuickLookUI frameworks only Apple can add support for playable QuickLook previews. There's some discussion of this under QLVideo Issue#3.

QuickLook's API is not all that great. It's meant for documents that can be displayed in a single frame, like text or photographs. There is no way that I know of (or apparently anyone else) to have QuickLook play movies. Apple obviously can, but they're clearly using APIs that the rest of us do not have access to.