Hi Badmother, wise words about the potential for knocking the derailleur, I do use my Brompton mainly for the commute and it gets folded and chucked around trains quite a lot. I guess there's a reason many people settle for the greasy fingers option. I have a 2012 3-speed and have worked out that with 44t and 38t chainrings and 13t rear sprocket I can get a gear range of 35"-73" with quite even 15% steps between gears with no overlaps. Funnily enough, thats almost exactly the same gear inches and step sizes as the Shimano Nexus inter-7 hub, but without the highest gear (84") and obviously without the simple linear shifting. I have the Nexus hub on another bike so I know I would like the range and step sizes if I got it to work on the Brompton.
I've been thinking (and thinking) about this idea for a while and I'm thinking you are right, I either need to take the plunge and go for it, or just drop it and move onto the next project! One of the delays is I have been waiting for the 2013 cranksets to become available. I don't want to go through the lengthy trial and error process of finding a double chainset that will give a decent chainline without compromising fold etc.

I'll report back here whether I succeed or fail. If it doesn't work at least I might save someone else the hassle later on!