I am just talking off the top of my head but doesn't Plan9 Support Posix? If it does then I would think the next step would be to port SDL to Plan9 since I know that Qemu for Linux uses SDL for audio and video support.
You other option would be to rewrite QEMU to use the native graphics and audio system of Plan9. I have been temped to port QEMU to GTK so that works better with XGL. I am afraid that life has not given me the free time to even start on that yet. You know things like a job, exercising, a wife..