The BAe146 used the cabin service doors as escape hatches. Each door was specially modified without hinges, and when unlocked a large pneumatic ram was used to push it out. Cabin pressure was dumped beforehand via a ‘dinner-plate’ sized butterfly valve in a nearby cabin window – practice cabin pressure dump was no great event, IIRC it took 20 secs or so.
The crew wore self contained parachute / oxygen systems with static line deployment. The rear door was the primary exit, avoiding the hazards of engine, gear, and extended flap, but if necessary, the front door would have been used.
The usual ground tests were conducted. The only significant ‘risk’ was the ‘run’ to the back of the aircraft guided only by a rope supported from the roof.
Latterly of course, the 146 rear passenger door was converted for para-drops; a pull-in and lift up-and-over mechanism. Initial tests involved sand bags thrown out from 50 ft tower flybys and then the intrepid ‘military’ para-testers from 10,000ft for real.
Many demo jumps were conducted in the UK, Europe, US, and Australia; IIRC some involved depressurization and jumps from high altitude – and some visits by people of whom you did not enquire their base or unit.
Later versions of the HS125 used a hatch/hole in the fuselage above the main wheel well. A clip-on static line parachute escape required the gear to be down, hatch up, and then a jump through the hole in the floor.
Prior to this design, various forms of a fireman’s pole or pogo stick from the main door were considered, also a pole for each crew member, rotating about the door sill which would have taken the crew out of the main door and below the line of fuselage / gear.