I'm happy to report total success with the Office Bag 2. The new mount arrived, with the little ear things (which are specifically for beam racks, it turns out), and I fitted it. At the front end, it was fine, at the back the rack sides were too deep, but I was able to fit it in a place where instead they can grab a horizontal piece. There's no heel strike and plenty of clearance, and it seems, frankly, just as stable when folded. I cannot now put 4 skate wheels on, as the ones at the rear of the rack would foul the case, but I could fit them at the other end and possibly roll on those, if I decide it's important.
Theoretically, I could now use the Bike Box, or indeed the Travel Biker, if I was inclined to spend Ortlieb money on such a thing again! I also have two mounts now, and I'd more or less given up using the Office Bag on my full size commuter, as having the weight high up caused me to snap P clips rather often. I'm thinking I'll fit it to the Twenty when I finally finish that, as the Pletscher rack is not much use for anything else.
Gringo_Gus - you might be right. I love, and have kept my Downtubes, and particularly the one I fitted with drop bars is significantly better on the road than the Brompton. I can get one in the boot of my car, or both of them and the Brommie in my bus, so they'll still get used. The Brompton though, the fold is precisely what I need for my particular situation. I still think it's too expensive, but I'm appreciating the quality of the engineering, and all the made-to-fit accessories (really with I'd got the dynamo hub!), and I do like that it's built in England.
Next thing will be a Carradice Camper. I've wanted one for a long time, and it would be useable on multiple bikes for multiple reasons. Whether I'd use SQR or not, I don't know. My current modus operandi is to ride with the lower part of the seatpost all the way out, and the upper out just the amount I need, and then I never fold the upper part. So far, it just about squeezes into all train racks that way. It's possible that SQR would fit on that portion, but it looks like it sticks up above the seat too. This means I'm tempted just to go with the loops.