I'm just "shotgunning" here to see if there is any validity to the concepts, so please take this in that vein.
First we find out if Zoombrowser EZ software comes with everyones' camera.
If so, then we can be generalists and create something that will work for everybody who wants/needs it.
There are USB "sniffers" that will look at a USB data stream and write it to a hard drive on a computer where it can be examined.
When a camera is plugged into a computer, there is a host-device handshaking that initiates certain actions. In my case, the portion of Zoombrowser that allows me to download/upload data from the camera starts up and is obviously in communication with the camera.
From this, if we use a "sniffer," we can learn the sequence of events involved when the camera is connected to the computer. This would enable us to build a simple stand alone hardware host - a black box - that can simply handshake with and open up the USB port on the camera, establish communications and prepare it for upload on the data lines, pins 2 & 3.
The box would have to have batteries to supply power to the camera as power is supplied by the host on pins 1 & 4.
That is assuming that there is no inbuilt host controller already in the camera as mentioned above as a possibility. If there is, that could make most of this much simpler.
This black box will also communicate with the USB Mixer setting it up to download/communicate and coordinate sending blocks of data and also providing power.
This data will be simply passed through our black box host to the camera. Our black box is little more than a traffic cop starting up both devices and acting as a conduit for the data.
Once the appropriate registers or memory locations are determined in the camera firmware, we should be able to use CHDK to write the incoming data stream from the USB Mixer directly to the SD card.
Again, let me clearly state that this is nothing more than conceptual brainstorming. I honestly don't know if any part of the above would work or not, just some ideas that might prove useful.