In response to a comment left on my original gearing post, I think I’ve found the solution to adding lower gears relatively easily. I was notified of the BromptonTalk group on Yahoo!, and searching there finds lots of stuff on dual chainrings.
The standard 50t chainring has holes drilled that will allow a 28t chainring with 86mm BCD. However, the bottom bracket is a bit short, and with the extra chainring, it won’t fold and the chain may rub against the frame. The solution to that is to fit a longer bottom bracket (BB). The guys at BromptonTalk have discussed at length the different BB options. The standard brompton has a 119mm BB, and, as far as I can tell, it needs about another 5mm for the dual chainring. The 125mm one of these seems to be recommended by people on BromptonTalk.
Voila, the gear inches for that set up will go down as low as 22″, with the two highest gears on the small chainring being similar to the two lowest gears on the large chainring. The manual change shouldn’t be too much of a problem to use occasionally.