First get xclass-xn-20000605.tar.gz, xrfbviewer-0.0.2.tar.gz, and patch-xrfbviewer.

tar zxf xclass-xn-20000605.tar.gz
cd xclass-xn-20000605
./configure
make

cd ..
tar zxf xrfbviewer-0.0.2.tar.gz
cd xrfbviewer-0.0.2
edit makefile so -L/usr/X11R6/lib rather than -L/usr/X11/lib
patch <../patch-xrfbviewer
make

Be sure you're running an X server with 32 bits of depth (alternatively, hack ScaleFramebuffer.cc to support 24 or whatever). Get or build an Xvnc (VNC can be found here), then

Xvnc -depth 32 :1 & (Xvnc defaults to 640x480 size)
twm -display :1 &
xterm -display :1 &

./xrfbviewer &

Under options, select preferred encoding "raw" and the scale option with 1/2 scaling. Hit OK and it should connect to the running Xvnc server.

Some screenshots. Note the degraded texture in the handhelds.org background due to the quantization to 4 bits.