Headless VA-API hardware accelerated H.264 video encoding with vaapih264enc?

[Matthew Thyer]

The Libav Wiki "Hardware/vaapi" page (https://wiki.libav.org/Hardware/vaapi) states that I should be able to use VA-API hardware acceleration without being connected to an X server.

Quote:
"The libva driver needs to be attached to a DRM device to work. This can be connected either directly or via a running X server. When working standalone, it is generally best to use a DRM render node (/dev/dri/render*); only use a connection via X if you actually want to deal with surfaces inside X (with DRI2, for example)."

It then gives an example on how to use the DRI rendering device directly with the "avconv" utility:
avconv -hwaccel vaapi -hwaccel_device /dev/dri/renderD128 -i ...

What's the equivalent for the vaapih264enc GStreamer element?
i.e., How do I make it use the DRM device /dev/dri/renderD128 instead of the X server?
I can't see any properties of the element that allow me to control this.
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