So I eventually figured out the answer to my own question. I had been running a light plugged directly into my Trek Ex-e, which worked fairly well if not delivering an enormous amount of light. I was planning on making a new light (since I have a home-made CNC mill) using 3 XM-L3 leds, as my initial research suggested that would be a good combo. I really wanted to avoid an extra switch on my bars and to make the light head super small by omitting the driver circuit, but alas, it was not to be. First, I realised that I was going to get less light output under-driving 3 leds than by driving just 2 in their sweet spot. Then, when I plugged an old set of lights directly into the bike without a driver circuit, it flickered, meaning the bike wasn't outputting the kind of regulated power that leds need.
For now I ended up taking the 12v XHP35 leds out of my most recent light and fitting two of the new XM-L3s (I think these are used by Gloworm and many others). I checked the set-up with a cheap bench-top power supply from Aliexpress. Set up at 12v and 2.5 amps gave me a power draw of about 16.6 watts (The bike will supply 17W). This gives a theoretical output of close to 3k lumens on full burn. More interesting was when I turned the light down to the mid power setting it was still giving a lot of light, but only drawing about 4 watts. If I turn it down to my commuting setting the power drain was barely registering. The driver circuit I'm using is an old Taskled b3flex buck driver, which is highly configurable, with a maximum output of 3amps, which unfortunately is more than the bike can supply. The next level down is 2.5amps, which pulls almost the full 17 watts the bike can deliver.
I have now had a chance to test it off-road, and I am really pleased with it. It is easily enough light for offroad riding, especially combined with my helmet light. I spent most of the ride with the light set on medium, and only turned it up to full for the downhills. Out of interest, I tried setting the max power to 3amps. It worked, but I don't think it was any brighter, and the bike sounded an alarm at me, so I set it back to 2.5 and all was good for the rest of the ride.
So: You can run a decently bright light straight off the bike without using the USB-C port. You won't get as much light, but you will get vastly more than the measly 1200 lumens the the factory light puts out.