I've just assembled a much more accurate repeater (though not 100% perfect, and I suspect that's unavoidable), based on the original design. While perfecting it, it looks like the rest of you have independently come up with a design using similar changes.
Instead of receiving water directly from a brook, it receives water from a pressurized tank (5x5, 15 Z-levels deep, filled from the brook), and instead of having a floodgate opposite the bridge to smash the water, it has a screw pump on the Z-level above which pulls the water off the pressure plate (and drains it off the map through fortifications) and a 2-tile drawbridge that lowers over the pump's input tile. This way, when the bridges lower, high water pressure insures that the plate is (almost) instantly covered with 7/7 water, and when the bridges raise, the screw pump will insure that the 7/7 water is instantly removed from the pressure plate.
While testing this version, I found that it operates with a period of 301-302 ticks, though very rarely it will take 303 - even with such high water pressure, sometimes it decides to take 3 ticks for the water to flow the 2 squares from the tank to the pressure plate. The actual timing of the device is as follows:
0 - water hits pressure plate, sends "activate" signal
100 - bridges raise and water is removed from pressure plate
200 - pressure plate sends "deactivate" signal
300 - bridges lower
301-303 - water hits pressure plate
Filling the intake using 2 screw pumps (instead of natural pressure) might remove the delay and give this device 100% predictable timing, though I haven't tried this yet and don't currently have the time to do so. Further experiments are welcome.
While working on this design, I also discovered that a retracting bridge takes 101 ticks to open/close while a raising bridge takes only 100 ticks.