ב''ה,
Take all this with a grain of salt, I don't even know how I met supposed morphine (specifically) fans in USA decades ago with it so tightly controlled, been around a fair few sick or injured people, no real experience aside from maybe in a couple surgeries where I wouldn't have known WTF I was getting or what was happening anyway if that was part of any surgical cocktail, and some codeine in youth as metabolizes "slowly" to morphine proper in the body.
Personally my general feeling is an opiate is an opiate is an opiate while they have subtle 'tequila vs. beer' kind of nuances in metabolism.
But when people complain of an "allergy" to something in the class it's usually morphine.
I think there's two major reasons, first of all it just hits so hard and fast I guess, that if the wacky histamine/cholinergic response is going to happen (itch and formication feelings some folks get with opiates) it happens more intensely and rapidly. Second if anyone does have some kind of plant compound allergy, maybe it's less removed from plant material, but I kind of think it's the former more often, see various references to opiate OD not being completely pleasant, goes from 'perfectly numb' to 'maybe kind of itchy and nauseous and aware something is wrong like slowed breathing but possibly too high to do much about it in the moment.'
So in practical terms if this is something you're trying to only do once, take a standard amount of Benadryl 40 minutes prior (25mg to 50mg, even if feeling 'YOLO' don't really need to more than double that) and hopefully it won't be an exceptionally itchy experience.
Now, in medical usage, oral codeine in USA is generally served with an as trippy or more toxic antihistamine for historical reasons to counter both abuse and the itch, and that may be why it has more a reputation for the 'opium dreams' kind of stuff than every other opiate, but possibly the itchy response takes like an hour to build up so if you're out before then it may not be too annoying.
In summary, seems like most folks I've met who have had reasons to say 'allergic to morphine' even if they were possibly fans of related meds had a story like 'got real itchy, maybe hyperthermic, and stopped breathing,' and that just is what every opiate in excess is going to do, but note that folks can find it dysphoric enough to have that opinion, so some Benadryl for the itch and maybe some benzos or barbiturates if you've got them to hasten the nap after doing what you're thinking of doing would add to the peacefulness.
If it does not succeed, and I don't have advice on dosage, risk of waking up with brain damage from hypoxia is probably the concern, and I gather that being jumped on with Narcan is life-saving but intensely dysphoric in the moment for fairly obvious reasons. Pharma is funded to hand that out like candy here 'to save lives' and whether you want that within reach yourself is your own choice.
As far as risks of addiction if you come out of it, a rare few folks become drug seeking zombies after their first taste and most, like with surgery, are probably just going to feel shaky and kind of gross for a week if waking up from an "OD" sort of experience.