Any chance of seeing the pic? I have operated with one man dinghy in a back pack, and from above and behind the seats this could look like a chute pack. Though I don't know if this as likely as a chute in a testbed airliner, I can only speak helos as thats all I've really worked on.
I wouldn't like to try jumping out of Concorde (but given the alternative.....), How slow can concorde go? There is also the wing and intakes to dodge.
Given the right conditions I suppose you jump out of any aircraft (emergency jumps have been done with helos).........though a chute would help.
I'm now curious about parachutes and airliners in general.....