Comment 7 for bug 1597437

I can confirm that the patch in #5 works. I've tested it on the same system (updated to 16.04.1 with all current updates), same video cards, same monitors, as described in the original bug description section above.

(long version:)

Re #5, thanks for trying this and updating with your result! You've now emboldened me to try this too, but you just say "I compiled the driver", you don't specifically say how :-(

So I've got the source folders with the apt source command you mentioned, applied the patch, and from the (patched) xserver-xorg-video-nouveau-1.0.12 folder, I've run .configure but I get the following:

[lots of "checking... yes" lines]
...
Package xorg-macros was not found in the pkg-config search path.
Perhaps you should add the directory containing `xorg-macros.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xorg-macros' found
...
Package xorg-server was not found in the pkg-config search path.
Perhaps you should add the directory containing `xorg-server.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xorg-server' found
checking if RANDR is defined... no
Package xorg-server was not found in the pkg-config search path.
Perhaps you should add the directory containing `xorg-server.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xorg-server' found
checking if RENDER is defined... no
Package xorg-server was not found in the pkg-config search path.
Perhaps you should add the directory containing `xorg-server.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xorg-server' found
checking if XV is defined... no
Package xorg-server was not found in the pkg-config search path.
Perhaps you should add the directory containing `xorg-server.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xorg-server' found
checking if DPMSExtension is defined... no
Package xorg-server was not found in the pkg-config search path.
Perhaps you should add the directory containing `xorg-server.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xorg-server' found
checking if DRI2 is defined... no
checking for LIBDRM... yes
checking for LIBDRM_NOUVEAU... yes
checking for XORG... no
configure: error: Package requirements (xorg-server >= 1.8 xproto fontsproto libdrm ) were not met:
No package 'xorg-server' found
No package 'fontsproto' found
Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.
Alternatively, you may set the environment variables XORG_CFLAGS
and XORG_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

Do you recall which -dev packages you had to install for this to work?
Okay google-answering my own question (for other readers)...
apt-get install xserver-xorg-dev x11proto-core-dev x11proto-fonts-dev

After this, ./configure and make worked, I moved the new src/.libs/nouveau_drv.so to replace the one in /usr/lib/xorg/modules/drivers/ as you said, and upon reboot, the 4th monitor works. Thanks! I've been sitting with it dark for months!!!

One concern though, is why is the new version 10x bigger than the old? Is the default ./configure settings set to leave debug flags on or something? and should I have done this differently?

$ ls -la nouveau_drv.so*
-rw-r--r-- 1 root root 2332552 Jan 13 12:34 nouveau_drv.so
-rw-r--r-- 1 root root 214960 Mar 3 2016 nouveau_drv.so.orig

But besides that, I'm thrilled. Please, developers, package managers, etc., help get this out to the less techy users out there.