OK, just bullring stuff:
Think of a hydraulic ram base at the bottom of the companionway. Its shaft when compressed is at bum height. Shaft extended is at top of companionway.
A simple, narrow, padded rectangular seat has one end secured to the top of the shaft.
Person hangs onto companionway rails.
As shaft extends the person can swivel the seat on the shaft to be facing aft so when shaft is fully extended he steps out into cockpit.
Understand what I mean?
Because the shaft rotates in the ram tube...
Sorry for non technical descriptions
But could be made at home, almost.