I did something like you ask for : a steel cable with one extremity attached to the bottom of the seatpost and the other to a round aluminum plate with a diameter slightly bigger than the seatpost diameter so that it cannot pass into the seatpost tube of the frame.

When the bike is folded, it hang under the the frame, when its unfolded, I choose the cable length so that the aluminum plate come almost against the bottom of the seatpost tube of the frame.

Of course, with tools, its possible to cut the cable but it prevent somebody to easily/quickly remove the seatpost+saddle with only opening the quick release.of the wheels and replace them by titanium 5mm axles that need an hex key to be dismounted.

There are special axle that require a keyed tool to be dismounted (from Pitlock for instance) that are safer but its made for expensive wheels, the Birdy wheels aren't very useful since they almost only fit for a Birdy.

You can see the round aluminum plate just under the bottom of the seatpost tube of the frame in the picture below (its the Birdy Touring with the Shimano Ultegra RD-R8000GS derailleur and Tune hub + 11s 9-34 Ethirteen cassette):