Can you drill a hole through the one corner of the square plate? If so then put an LED on one side and a detector on the other side. The LED would shine through the hole for a short moment as the hole turns by and allow the detector to see the pulse. To increase accuracy and reduce outside disturbance you can mount the detector in a small tube that has an inner diameter similar to the hole that you drill through the corner of the plate.
You could compare the diameter of the hole to the circumference of the circle traced out by hole as it rotates around. That ratio compared can be used to compute the pulse width to expect out of the detector considering the rotational period of the plate. At 500Hz the rotational period is 2 msec.
Place the hole in 0.5 cm from edge of each side of the corner of the plate so that the of the plate gives the hole a rotational radius of 1 cm. This will allow the detector tube to be fully obscured by the plate except for when the hole is directly over the tube. At 1cm radius for the circular path of the hole the total circumference would be ~6.283 cm. A small 0.3 cm hole would result in ratio of 0.3 / 6.283 = 0.0477. The scaled rotational period would make the time the hole was over the detector tube be on the order of 95 usec. Of course the sensor would see a increasing and then decreasing amount of light from the LED as the hole eclipses over the detector tube.
Any reasonably decent photo detector should be able to "see" a pulse eclipse that is 95 usec wide. To make the circuitry easy to deal with I would recommend that you try a detector such as the Fairchild QSE158 available from Mouser for around a buck. A detector like this makes things easy because you simply power it from 5->15 volts and get an open collector logic level output that you can interface to your MCU with a suitable pullup resistor.
An emitter that is compatible with this type sensor would be the Fairchild QEE123. Also available from Mouser this part can biased off your supply with a series resistor selected to permit from 20->75mA to flow through the infrared LED.
A couple of other things also come to mind. You could put the hole very near the corner of the plate but then the detection logic of the target MCU becomes a little bit more complex because you would have to discern the difference of the plate corners passing over the detector versus a corner that includes the additional narrower pulse due to the hole.
If you cannot drill through the plate then consider gluing a piece of opaque plastic tubing all along one edge of the plate. Then put both the emitter and detector each at ends of an equivalent piece of tubing that is lined up with the edge tubing once per revolution of the plate. This will give a similar eclipsed pulse scheme. Due to the greater distances involved you would need to use a bit more care in lining up the tubes and aiming the sensor/emitter concentrically down the center of the tubes.