All I did was to replace the /usr/bin/X11/X with XF86_WYSE tried restarting
the X server. At this point linux complained about a couple of missing
libraries, which I found and installed with dselect quite easily.

Restarted X and got a nice display on the T1500.

To get the mouse working I changed the XF86Config mouse device from
/dev/gpmdata to /dev/mouse (or /dev/psaux) and stopped GPM.

I installed twm from the debian packages and added 'twm' to .xinitrc
for the user I run X as.

So, in short:

Download XF86_WYSE
Replace /usr/bin/X11/X with the above
Change the mouse device to /dev/psaux in /etc/X11/XF86Config
Change the Horizontal and Vertical frequencies for the monitor
Stop GPM
Install twm (or some other window manager)
Add 'twm' to .xinitrc in /root or /home/whoever
Try starting X
Install any missing libraries
Start X backgrounded: startx &
export DISPLAY=localhost:0
run an xterm 

Et voila.

There still seem to be some issues:
Dragging a scroll bar seems to send the mouse crazy and I have a few
lock ups in X.

XDM ought to be easy but there are problems with xdm and Debian. Its
easy to install and start xdm but after logging in nothing happens.
The logs show that nothing is authorised to connect to the X server.
No solution that I know of so far.