Depending on the local laws, any suspicion of the death being a suicide may for example mean that the organs are unsuitable for transplant anyway, or there may be other rules that make it practically not an option, for example if an autopsy has to be performed if suicide is suspected, that would practically also mean no donation.
So it may be different in different countries, but I guess in most of them it would mean making it look like an accident.
But accidents involving for example vehicles have significant chance of causing harm and injuries to others, which doesn't follow the spirit of your noble goal.
Also, painful and gruesome methods have a significant risk of failure, and a failed attempt makes a lot of resources will be used on the one who has attempted. If you're trying to fix something you think you have messed up, like make your death meaningful through the donation, it doesn't exactly work with CTB very well, huh?
You likely COULD donate your body to science (check your local laws). If they have to do the autopsy, they do, but once they are done with that the students can practice on it, or parts can be preserved in jars, bones used to teach anatomy etc.The organs don't "live in someone else", but still are useful. Also the family may be spared the cost of the burial (at some places once they are done with using the bodies for education they make a fancy burial with the families and the students and it can be much more fancy than what a "donors" family could afford on their own). Still of course we're on this site because the medical profession can be very oppressive and harmful for the patients. A person who wants to do it in an "extremely altruistic" way would choose some old and well-researched method, like hanging, because seriously one more or one less hanged body shouldn't make any difference in the medical understanding of this method. Doing it with nitrite and donating the body could help prevention of this method, and you may prefer to not contribute to depriving people of options.
This is purely theoretical, I'm not the "omg, don't ever fart, farts contain methane which harms the environment" sort of person