there are wings that hang below beam racks to stabilize a small pannier ,

and even some bags made with 'spaniel ears' for more capacity ..


the scheme some people used was stand a Backpack up

on their Brompton Rear Rack.. and secure the bottom of it far enough Back
so as to not Kick It.

Another Bromptonaut , Used the front Touring bag and a Camper long flap
Carradice saddle bag .. without a rear rack ..

I snagged A Carry Freedom City.. Bike trailer before the Scottish company
changed the distribution deal with their only US retailer .

It folds FLAT . bag comes out with 1 buckle.