My understanding is that that can't be done: the relevant pin is only present in the special 5-pin connectors and is not wired through to the other end, nor is it available in normal USB connectors (which only have 4 pins).
This issue of not having to carry 2 cables around means I probably won't bother to buy a "host mode" cable even when I can get one. Using the Nokia cable plus a gender changer and controlling the mode from the command line means I only need one cable (plus the gender changer) and I can either use it to connect devices to the N810 or use it to connect the N810 to my laptop (for example when I want to download a video, convert it on my laptop in my hotel room and copy it to my N810 for watching on a plane later).