I consider it a point of natural law that all people are entitled to the fruits of their labor, and aren't entitled to lay claim to another's labor.

Your natural law is a central tenant of more than one branch of libertarian socialism. Literally, some hold the idea of if you make it, you alone get the first decision to keep, use, sell, etc. As opposed to a wage based system where the owner is entitled to the fruits of your labor in return for some wage determined by market forces.

Socialism asks people to work for other people before themselves, and as Rand clearly showed, such a system is inherently morally hazardous. Essentially, altruism enforced by force is an expression of sadism.

I tend not to think of enforced altruism as altruism anymore. That said, most societies have enforced people to work for the benefit of others. This can be done through either hard force (violence) or softer force (economic pressure, social censuring). A better place to start I feel is how much effort you put forward vs how much benefit you receive. Also, if we were to play to the anarchic ideal end of libertarian socialism, there would be minimal hard force which could be applied to anyone.

That said, there are a ton of problems with getting a socialist society to work in reality, so I would still consider it a pipe dream currently. Nonaggresive forms of society tend to fall pretty quickly to more aggressive ones in an unprotected environment. I could see problems with insufficient investment and technological advancement plaguing a decentralized socialism system. Also, hard force may be less than ideal, but it can be effective at keeping a society ordered and productive (although it can backfire if not applied sparingly and carefully). Maybe I am too cynical, but I think a major reason markets work so well is because of the threat of retribution if we do not play nice. It is not that I think everyone won't play fair, however I think enough people will lie, cheat, steal to bring the system crashing down without that potential retributive force.