I'd love to see GPU support for Plan9. This discussion falls right into my professional capacity. I'll say that people generally *grossly* underestimate the complexity of a modern GPU and of its supporting software stack. The GPU driver is effectively a second operating system with shared memory and DMA interfaces to the host. Even bringing up a modern GPU for just compute tasks is a very large endeavour.
That being said, if you want real hardware support, the best place to start is currently AMD's open-source stack. Ignoring the Vulkan bit, understanding their platform abstraction layer (PAL) and shader ISA (
https://developer.amd.com/wp-content/resources/Vega_Shader_ISA_28July2017.pdf) is the base. The lower hardware levels are reasonably well-described in linux's libdrm and its AMD support in amdgpu.
Opinions on how to bring this to Plan9? I don't really have any - it's a huge pile of work with minimal benefit. If you're looking for lightweight graphics, WebGL is a doable path, and almost certainly the right way to experiment with Plan9-like interfaces to graphics hardware.
Paul