Lots of good ideas here.
Regarding the hinged knee that Captain
465 mentioned, if the extension need not be raised or lowered frequently, the knee could be bolted to the vertical surface (if indeed there is one), and removed completely when extension is lowered.
A removable support leg could be utilised; I did this using chromed brackets made for this purpose. Thumbscrews to the leaf underside and to the floor.
The outside the box solution is to have lines with snap eyes on the ends attached to each corner, and strong padeyes overhead. Obviously results in obstructions above the table, but none below.