The following works with libnx without setting up a No Machine
style NX server.

0. Get the magic cookie for your local X session

COOKIE=`xauth list | grep ${HOSTNAME}/unix:0 | \
grep MIT | cut -f5 -d' '`

also decide on a port

NX_PORT=80

and create entries for this port

xauth add localhost/unix:${NX_PORT} MIT-MAGIC-COOKIE-1 ${COOKIE}

xauth add localhost:${NX_PORT} MIT-MAGIC-COOKIE-1 ${COOKIE}

1. Log in to the remote machine and run

NX_ROOT=${HOME}/.nx
mkdir -p ${NX_ROOT}/C-${NX_PORT}

NX_HOST="nx/nx,link=adsl,stream=9,pack=16m-png-jpeg-1,nodelay=1,root=${NX_ROOT},log=session,stat=stats"
NX_DISPLAY=${NX_HOST}:${NX_PORT}

xauth add localhost/unix:${NX_PORT} MIT-MAGIC-COOKIE-1 ${COOKIE}
xauth add localhost:${NX_PORT} MIT-MAGIC-COOKIE-1 ${COOKIE}

/usr/lib/nx/nxagent -nolimit -display ${NX_DISPLAY} \
-class TrueColor -noreset -geometry 800x600+0+0 \
-auth ${HOME}/.Xauthority -name 'NX' \
-bs when_requested :${NX_PORT} &

DISPLAY=:80 fvwm &

2. Back on the local machine you run

NX_ROOT=${HOME}/.nx
mkdir -p ${NX_ROOT}/C-${NX_PORT}

NX_HOST="nx/nx,link=adsl,cache=4096k,accept=127.0.0.1,root=${NX_ROOT},log=session,stat=stats"
NX_DISPLAY=${NX_HOST}:${NX_PORT}

ssh -f -N -L 4080:localhost:4080 as82

/usr/lib/nx/nxproxy -S localhost:80 &

The No Machine screen will popup after a short while and you will get
your usual fvwm setup from the desktop.

The difference (in performance) between this and vnc was not noticable to me.