Hi, I looked into QXL over a year ago. I've never had the time to write a driver for it, and these days, virtio-gpu might be a better target. If you're mostly interested in the ability to control screen resolution, you could try the VMsvga2 driver with Qemu's VMWare vga simulation. I believe the driver might need a tiny modification to work with Qemu's variant of the device, I think I can link you to the correct branch if necessary.
Virtio-9p might not be too hard, I noticed a 9p implementation for OSX/macOS a while back. We now have a generic virtio stack (to be published soon with the block, scsi and memory balloon drivers) so it might be possible to plug the existing 9p driver onto that.
The virtio stack would obviously also make writing a virtio-gpu driver a bit easier.
More Qemu guest drivers for Mac guests are mostly a question of allocating our time - occasionally, someone will pay for development, otherwise we work on it if we have a specific need.
Hope that helps