*Update: I just went ahead and tried to implement this with the Arduino Uno that I had at home. I simply connected the wiring for pins 2 to 9 on the db25 connector with their counterparts on the UNO (digital pins 2 to 9) and a single ground on db25 pin 18 to my Arduino Uno ground.
then I used @tstenner’s code with the nice bitwise operator function (took me a bit to understand how that works … really efficient!) and hooked the interface up to the EEG equipment.
Using python and pyserial, I could send bytes using port.write(bytes([1])) and it worked like a charm 
The latency on the Arduino Uno is about 3ms for a roundtrip so I am ordering an Arduino Leonardo now to get sub-millisecond precision.
I’ll do an in-depth write-up of this and share it as a tutorial somewhere. Thanks again!