To connect a coin-operated switch to the LaserWriter, try setting the paper
tray sensor switches to a "tray out" condition (i.e., no paper tray in the
LaserWriter). When a coin is inserted, check for voltage at the paper feed
rollers' solenoid (this way you know a sheet of paper has been ejected from
the paper tray). Then, reset the the printer to the "tray out" condition
until the next coin is dropped.
The connections for the three paper tray micro-switches are at locations
J214-2, J214-4, and J214-6 on the DC controller board. The paper feed
solenoid is connected at locations J210-3 and J210-4, also on the DC
controller board.
Keep in mind that any modifications to the LaserWriter will void any
warranty or AppleCare coverage.