Hey all!
Wasn't really planning on sharing this, but it's done, and I'm actually kinda proud of myself, so here you go.
I'm using QEMU on ubuntu studio 16.04. Windows 7 has 4 cores, 16GB RAM, and a GTX 760 passed through.
The startup script also starts the synergy server, and the synergy client starts automatically on Windows. The virtual C:\ drive is on an SSD, and there's a second virtual drive on the HDD. Also set up a samba folder for sharing files with the host.
I also made a second startup script which assigned fewer cores and skips the GPU passthru for non-gaming tasks (downloads, updates, etc.). This leaves both the host and guest in a reasonably usable condition. Finally, there's a script for restoring the virtual C:\ drive in case it's corrupted (which I've even had the pleasure of using now, twice, because &$^%@ steam).
This has honestly gone way better than I imagined it would. Games are performing wonderfully. The only really frustrating part was getting a USB hub passed through (this took WEEKS!). The GPU passthru went perfect, first time. I still have a few tweeks to make. For example, the drives are currently using IDE drivers, so I'm going to switch those to virtio.
This is my son's rig, and it will be going back with him when this visit is over. But it's been a blast.
Thanks for looking!