I mean, sure, they're different things but I think the point still stands. You wouldn't recommend running in front of a bus because it's a bad idea. That logic applies to recommending anything, you don't recommend it if it's a shitty idea.
I don't think that there should be a keep inventory perk. Scamming people is a big thing with prison-like many people have already mentioned before me.
For how many perks there are already on prison, adding a keep inventory one feels really pointless. It would just be another perk which is very limited in usefulness, which would make people's chances of getting an actually good perk from keys even worse. I don't even get how you can die in lava as long as you wear armor, since most of the kit armor makes lava tick damage take ages to kill you, and emerald+ have /fly to negate fall damage. All things considered I feel like this would not be a good addition.
I don’t play prison but I don’t like the idea of this, it would most likely cause anger in players and cause arguments.
I think the only thing that would make this "fair" would for the perk only to work on your plot(s) but even then it wouldn't be worth it. Part of prison is pvping and scamming so you have to deal with that even if it's sucky. The perk would ruin that aspect...