Don't use parts others Covet?

given its Brooklyn .. move .. ?
the old country, Breukelen NL . near Utrecht , might be Nice.

with a common seat post which is a tube stepped down to take a common saddle clip.
you could seek out a piece of cable make a loop in it over the saddle rail, run that cable
down thru the tube seatpost,into the frame..

then IDK, make another loop, then re assemble the BB so the axle goes thru that loop.

the water bottle boss bolt, can also be used to anchor something on the inside of the frame.

just use a longer bolt.