Thanks very much, Dan!
I'll look at those racks in more detail and will try to imagine how they might fit my bike, along with any potential problems. It looks like at least one of the racks isn't currently available.
I'm also wary about clamping anything to the frame tubes. I had thought about using something on the top tube, but wasn't sure how well it would work, and if it would need also need supporting in additional dimensions (ie to stop the rack flexing or twisting around the frame). There's quite a lot going on in that area of my bike, with the steering tube, brake hoses, and headset etc., not to mention my feet! The light mount could probably be removed if I fitted the light to the rack, although the light wouldn't then move with the steering.
I had also thought about using the three bosses on the top tube, but I'm not sure how strong they are. When I tried to remove the bolt from the first one (nearest the head tube) the rivnut slackened in the frame. I've been able to tighten it up, but I'm a bit wary of cranking down on the bolts now. Also, I'm not sure how thick the frame tube is in that area, or if there have been any problems in the past (such as corrosion).
I haven't been able to find any pictures of factory-fitted racks for this type of bike. so I'm not sure if or how they might have been installed back in the day.
It would be nice to have a bit of weight over the front wheel when touring, but if a rack and luggage might cause a problem then I'll leave well alone...