Low tech solution with just some spanners, wrenches
My use case was removing seats from a bus. These were bolted in through the floor into nuts below; whole nut-bolt assembly rotated together. Nuts and thread below naturally quite rusted.
My bolt heads were hex, that were NOT flush into the cabin floor. This meant I could lay a spanner flat on the floor, holding the bolt head.
Now, if you have a spanner or adjustable wrench or locking pliers long enough that its handle can strike some immovable fixture above when turned clockwise in the cabin i.e. at the bolt head, then turning the nut counter-clockwise below will turn the spanner clockwise above, so that it strikes and pushes against that surface, which could be e.g.
- a wall or wheelbay edge
- a pillar of the seat assembly itself
- or anything else solid enough to prevent the spanner turning further clockwise
I'd lay my spanner flat on the floor with ring end over the bolt head, and placing its handle just left of (counter-clockwise of) a surface that would stop its rotating clockwise, then drop a heavy object that won't slide off, e.g. a sandbag, on top to keep the jaws of the spanner from popping off the bolt-head. Then I could unscrew the nut counter-clockwise, below the vehicle.