Ok, so here is the verdict:
(drum roll....)
You can't. There is no way. I have however after a bit of fiddling figured a solution that proves as good as, and reliable : a good USB splitter and a very, very long USB cable.
The splitter is on the mount side and is connected to guide camera, dslr and HC (via serial/usb adaptor), and the splitter itself is plugged on a massive USB extension cable. Unfortunately, there is no way to do without the HC, HOWEVER having only the usb extension to pull inside, up to the office (15 meters) is much less of a hassle than trying to extend everything including that pesky phone cable.
It must be noted though that as it turns out, it is really imperative to have a "good" splitter (branded, USB3.0) and a "good" cable, properly insulated as the cheap ones I first tried gave me poor, unstable connections with data loss and corruptions (especially as I send the DSLR pictures directly to the computer, not using a SD card). My theory is that cheap ones are more prone to interferences.