The weight saving of the Ti rear triangle is reasonable in absolute terms, but very poor for the money. It is a nicely-made part but leaving off the pump peg is bizarre. Unless Ti Brompton owners always hail a taxi when they get a puncture, they're still going to have to carry a mini-pump or CO2; and those sneaky gits at Brompton include the lack of a pump in the quoted weight saving!
You can stop a steel rear triangle rusting from the inside, which is the usual problem, by squirting Waxoyl aerosol into the tubes, which are helpfully open at each end. The brake bridge can be done through the brake drilling. It adds no real weight and should work for a few years before you need to do it again. I injected NSTN's old Brompton, which was pretty much rust-free*, all over with Waxoyl.
*same goes for London cars; the gearboxes and brakes may be shot but they don't rust badly because the roads aren't salted as much as in the provinces and they dryy out quicker.
