It turned out that the beam spot is quite visible even with the lamp on. But, for posterity, here are the instructions anyway:
Point your browswer at the hareboot28 server.
Login with the password for the "hareboot" (or "IOC reboot") servers posted on the wall.
The target illumination lamp is on socket 8. Select that socket with the
checkbox, choose the appropriate item from the Action" menu, click
"Next>>" and follow the instructions. (Figure 1 shows and example of the
main screen.)
Figure 1