yahoo brompton group has a how to series of pix, on drilling out the poprivets. then narrowing the aluminum tubes , and clipping an Ortlieb pannier on..,

UK has some people, who modify brompons, make racks ..
Kinetics in Glasgow has a rack to fit a pannier pair.

Steve Parry, also in UK, made a clip that lets the HB bag
get screwed onto it.

he also made a tandem that you can store in a closet..
Starting with 2 Bromptons..