Check the seatpost diameter.

Some brands of seat posts have a side mount option that should give you a full 360 degree rotation. Not that you necessarily need the seat upside-down.

This is a pretty large in diameter post, but an example.

https://www.ebay.com/itm/specialized...gAAOSwCBtbeadq

The problem they may not be as strong for holding the seat in place as some of the newer 1 and 2 bolt posts.