Brompton owners

I'm making stuff up as I'm going along here, but what's to stop you from drilling the frame and tapping it to accept a grease nipple.
Then pumping it with grease, like pumping with grease was an Olympic event.
Just a thought....

Keeping the area around it clean, regular folding and unfolding and a drop of oil now and then goes a long way.

Any way of maintaining the hinge on the rear frame, or is it something which is just left to wear out

Nope, that part is designed to wear, Brompton recommend getting it replaced once a year base on folding twice a day, five days a week.

It base on how often you use it, some people last for a couple years, some a mere 4 months, my advice is to simply keep it clean and get the Brompton serviced regularly. I strongly do not take @JurekB advice on drilling the frames at all.

This job is best left to mechanic with Brompton experiences/training, the average labour price is £45 (it's usually a 45 minutes jobs) and the hinge themselves are £20.

Again please pretend @JurekB is a fragment of your overly active imagination.