I managed to get a cheap dynamo mount from eBay. Generally these mountings are not designed for the sort of forks found on the Yuba Mundo (the same as on rigid mountain bikes), favouring the thinner fork tubes seen on bikes like the Raleigh Tourist or Twenty. Luckily I managed to find some longer bolts in my box of fixings, and the bottle dynamo is now mounted on the rim.
The snow has died down a bit now, so I have returned the Yuba to its normal configuration and taken it out for a test ride. The dynamo works fine on the rim, and the drag is negligible. The main difference between this and the hub dynamo is the gentle whirring sound and the vibration it sends to the handlebar. Whilst I can see now discernible difference in my speed for a given effort, the psychological effect of the sound and vibration means that the bottle does feel like it drags more than the silent, vibration-free hub.
Overall I am happy with the set-up and would recommend it when a hub dynamo is unfeasible.
UPDATE: Don't wrap dynamo wire around the hose of a hydraulic brake. At high speeds the electricity seems to magnetise the steel layer inside the hose and slightly activates the brakes. This could just be a peculiarity of the dynamo/light/brake set-up I am using, but I advise the other 2 or 3 people out there in the world who want to run hydraulic discs and a bottle dynamo on the same bike to be aware of this.