>> how about fabricating ? creativity..

you take out the BB and drill a hole . say just above it..

tie a cable up inside your seatpost.. plain ones. that can be the top of it ..

you pull the cable out the hole in the BB area , and tighten it with a screw .



then the seat post will only come out as far as the cable will allow.

**
It was a consideration easily done with a Brompton Telescoping seat post , because the main post is flared at the bottom.


and the rubber bung in the bottom uses a screw wedge to tighten it. and the upper post is a standard straight one .