Yup, the Carradice Bagman was part of my list in case I couldn't find anything else, but I'd rather keep the weight as low to the ground as possible.
Thanks for the tip on the tiny front racks. I'll take a look.
Another possibility I thought about while walking around a big sports store is to simply tape a mini basketball A 20cm-wide basketball should be wide enough to hold the Ortlieb and I could deflate it a bit so that it sits ok on the bike's frame.
2011 Raleigh Sojourn, 2012 Marin Four Corners, 2013 Soma Saga
I looked around a bit for you, out of my own curiosity. I found this item, which is pretty cheap (probably in quality as well as price) but it's a seatpost rack where the platform and the support are separate parts, you could easily slide the rack platform as far forward as possible and it looks as though it would nestle nice and close to the seatpost if you wanted it to. Then you could just hack off the excess.
But before I'd go to that extent, I would try to just strap it in place without a rack to make sure it doesn't interfere with pealing/seating. In your image (in which I assume you copied a stock photo of the bag onto a stock photo of the bike, which are likely not in the same scale) it looks like the bag may be too close to your heals on the upstroke and too close to the back of the saddle to be comfortable. Good luck.
I keep editing this post because I keep finding things...the one linked below would likely be good too.
There's a couple that toured on the Brompton and their method of attaching the rear bag was very good. Actually, they used a regular backpack!
First, you purchased the wrong Brompton and need the version that comes with the rear rack. Maybe you can still retro-fit your Brompton and install the rear rack. The Brompton rear rack is slanted and low but it's still useful in keeping the weight off the seat post.
Also, you'll need to change the rear suspenion block to one that's harder.