I wanted to chime in with this since I followed a solution used by others with great success. The problem with using simple audio splitters to daisy chain from one camera to the others is that one slightly loose connection or out of tolerance connector will throw off your entire system. It is very easy to get loose connections since there are so many of them (up to 5 possible loose connections per camera if you have to use 2.5mm to 3.5mm adapters), and the cords will be putting strain on the connectors, causing premature failure.
Because of this, you'll have a hard time diagnosing which connections are bad since all the cameras will be connected to a single line. When that line's signal is disrupted by a poor connection somewhere down the line, it feeds back to all the cameras at once causing them all to act funny, making it difficult to diagnose the source of the issue.
The solution to this is using an opto-isolator, which isolates each camera's remote signal so that they cannot feedback to the rest of the cameras. If one camera is acting funny, you can tell which one it is and fix the problem. The issue is that you will need to have someone who knows electrical design who can design and make you a board that can take a signal in from your corded remote, and split it to the optoisolated cameras.
I have designed an optoisolator board that takes in a 3.5mm signal from a remote and splits it to 8 isolated cameras, specifically for use in photogrammetry. I use 3.5mm because it is the cheapest audio cable to buy. ($0.88 for a 6ft 3.5mm audio cord at monoprice.com vs $3.31 for a 6ft 2.5mm audio cord) Then I use a $0.15 3.5mm to 2.5mm adapter to connect it to the camera.
Unfortunately, I've only made prototypes for my own personal rig so its nothing fancy. It doesn't have an enclosure, just an open board, which still functions perfectly fine. Its not at all delicate or anything. I could make some for you, but since its a low volume thing that I'd have to put in labor to build, I'd have to charge something pretty steep for it, like around $100 a board + S/H. If I had more of a demand for the board, I could probably lower the cost a lot, as it would be much cheaper to manufacture. Let me know if you're interested.

Brian