If a person were to take 150mg of white and presumably pure mescaline hcl at 10am, would they have any hope of getting to sleep by 12 that night at the latest?

Everybody has his own biology so it might vary from one to one but, for me, 150mg is a low threshold dose and IME 12 hours after ingestion (empty stomach) i'm sleeping (though, weed prolly helps in my case).
I repeat, some people are more sensitives.

Well, 150mg is not a very high dose so for me I'd be sleeping like a baby around midnight especially if I have some good bud to help with the comedown!

That may just be me though!

Edit: Not just me after all!

Let's say the person happens to be one of those who are sensitive to mescaline. Would they have a chance of sleep at that dose?

It all depends on how your body reacts to mescaline.
If you need sleep for work or something i`d hold off until a better time.
Personally I have taken cacti brews several times at night that were pretty underwhelming. Usually even if I only get just beyond threshold effects it does very little aside from keep me awake for 10+hrs & make me feel a bit off.

In the event you do dose, you have a 50/50 chance. Either you will be fine & fall asleep in time, or you won`t & the next day will be rough.

How adverse are you to taking other medications to help with sleep?
Mescaline is usually considered to be a 12 hour trip, so you should be mostly back by 10/11pm.
If there is residual stimulation, benadryl, alprazolam, cannabis, or melatonin all should knock you out.

If you're opposed to that kind of thing (as a rule, I like to be substance-free for a while after a trip, for example, and don't take things for the comedown), I would say give yourself more time. Why do you need to be asleep by 12? You're still likely to be fuzzy the next day - many psychedelics have a bit of a cognitive hangover (eg: I usually have difficulty focusing of a day or so after dosing). If you have something to do one day, don't go tripping the day before. You might be off the next day, and anxiety about performance could color your trip.

Also, we don't do all that SWIM/hypothetical stuff here. I use drugs, you use drugs, it's not anything to be ashamed of. It's a forum for psychedelic enthusiasts, you can own your interests.

Blessings
~ND

Yeah I'm also against using substances other than the one I originally intended to take, especially ones like that. I guess I have some things to consider before moving forward. Thanks all for helping put it into perspective.