Okay, then, here you go:
1. For the hackers:
a) Use the hpress module (you'll find it somewhere here) to trigger video recording via half-press.
b) Connect a 2.5mm jack (on some cameras, on others it might be the special trigger-connector) on the camera. The first line is GND, second is half-press and gets pulled low when half-press button is being pressed. The last is full-press, we don't need that.
c) Use an Arduino to measure if half-press is low. For simple purposes, just connect it to a digital pin and do some digitalRead. For the more advanced users: use external interrupts, that should allow lower delays.
d) Now get a 2.5mm jack (I think the jack on the Zoom is 2.5mm) with 4 lines. I don't know the order, but I think the 4 are: Vcc, GND, RX, TX.
e) Connect correctly, check if voltages differ (It might be everything, most likely 1.8, 3.3 or 5V). If voltages aren't the same between Arduino and Zoom, you'll have to use level shifters. Now a record message can be send to the Zoom. Done.
e) Optimize the circuit by using some ATtinys instead, start a kickstarter and get rich.
2. For the rest of us
a) wait for 1.e
EDIT: it's 3.3V - luckily ATs run that stuff
EDIT: also: the other way around is also possible: make the zoom start video recording. It certainly sends a message when starting. Pulling the half press low causes the camera to record as well (when the hpress module is loaded, off course).
