a spacer added between bag and seatpost is easy to fabricate.. just need a longer strap

you might just get a piece of plastic PVC pipe. run the strap thru it.

Or maybe 2 , one to secure the stand off to the bag, #2 to strap the standoff piece to the seat post..
shopping only solutions?
Look at the carradice Bagman frame then.. it clamps onto your B17 saddle rails,
or the new standard Brompton saddle, if you don't make it so Pushed forward.

the saddle provides the bag loops.. but it will still be there sticking out back
when the rear is folded under , and the seatpost pushed down..

I'd just put those things in the Touring bag, up front, instead.
I use 1 rear pocket for that stuff, the other for water..