In emergency situations I'd think that a permanently mounted stern ladder
would ultimately be more reliable/usable/quickly deployable than any use-then-store ladder
arrangement... we've had both over the years.
The main problem with most off-the-shelf ladders
is that the angles won't match the transom exactly, or the steps/rungs won't go deep enough. There are some nifty collapsing ladders
that fold down and extend deep but they work best on the short vertical sterns of the more recent open transom designs (such as that stock on the Bene 36.7)
I can't over emphasize the convenience and safety of a DEEP ladder. If the ladder doesn't have at least 3 rungs underwater it can be very difficult for out-of-shape individuals to clamber out of the water - even in a calm anchorage and non emergency situations. There's also a lot of strain on too-shallow ladders as the climber strains to get out, most of the effort is lateral rather than vertical because their feet are so high relative to their body. We had an aluminum over-the-side ladder fail this way.
All of which leads to the idea that this sort of thing is best done by custom fabrication.. a bit costly perhaps but this can/should be viewed as a safety item.