I have used this and would not hesitate to use it today in high scene brightness ratios.
I don't stop my lens down five stops, however. What I do is to meter through the opaque acrylic panel and then determine a Zone III placement. I next place the acrylic panel over the camera lens and give the film the first exposure at that Zone III (through the panel) exposure. I then give the film a normal exposure for the second exposure.
This effects of preflashing film is best illustrated by the following example. If we take the premise that each zone or stop of exposure represents a doubling of light. Then we start out as Zone I having a value of one, Zone II having a value of two, Zone III having a value of four, Zone IV having a value of eight. This continues until we arrive at a number of 128 for Zone VIII.
By preflashing film in this manner we raise our Zone I exposure to a Zone III 1/3, our Zone II exposure to a Zone III 2/3, our Zone III exposure to a Zone IV, and our Zone IV exposure to IV and one half. The addition of four units of light are miniscule in relation to the 128 units that a Zone VIII would normally have. Additionally this allows us to place our low values at Zone I and compress the brightness ratio by a couple of stops. The compression of values will occur in the shadow regions as opposed to the highlight regions if we were to try to accomplish this at the printing stage by preflashing the paper.