Servers where some GPUs have displays attached and some don't is trickier than the fully-headless case. The first and obvious issue is that some display IDs are already occupied. That's fixed by picking displays :10, :11, :12 etc, which aren't commonly used. Then the script will actually run!

Unfortunately, it also blanks your physical display, presumably because it's nicking the GPU off of your primary X server. This is fixed for by only looking at GPU buses that nvidia-smi reports as 'not displayed'. That leaves these problems:

• How'd you figure out which physical display IDs correspond to which PCI buses, so the script can manage those too? This seems easy but Google's failed me so far. xdpyinfo seemed promising, but the extension that presumably has the bus info in - NV-CONTROL - isn't supported.
• When launching X servers for non-displayed GPUs, the monitor will blank and you need to hit Ctrl+Alt+F2 to get back to the desktop. I think this is something to do with X 'resetting' VTs, because the same problem was originally showing up every time nvidia-settings was called. That was suppressed by -novtswitch and passing a new VT ID, but the blank-on-launch persists.

So yeah, if you've got a partially headless box and want to fix this up

git clone https://github.com/andyljones/coolgpus.git
cd coolgpus
git checkout partial-head
sudo $(which coolgpus)

and Ctrl+Alt+F2 back to your desktop.