Three mechanisms by which prosocial actions could be selfishly rewarded:

1. Correlation. You acting prosocially is correlated with aliens in other universes (and other humans) acting prosocially, which is good for you (especially if your preferences are scope-sensitive or not-super-indexical).
2. Some future humans may want to reward people who prosocially made the AI transition go well.
3. 1. BOTEC: 2% of lightcone will be gifted to people who made the AI transition go well. Donating $1 well now gives you 1/60B of the credit for making the AI transition go well, if it does. Therefore donating $1 well now gives you 1/3T of the lightcone, if the AI transition goes well, in expectation. But assume a 2.5x haircut for funging, so 1/8T.
3. Some aliens may want to (acausally) reward people who act prosocially. (Aliens could reward people directly, based on actions of correlated simulated people, or they could (more simply) acausally cooperate with future humans to get future humans to reward prior human prosocial actions.)
4. 1. Or, generally: maybe part of the multiverse-wide acausal cooperation scheme will be rewarding prosocial actions, alongside other incentive-compatibility measures like punishing threats/conflict.
   2. Or, imprecisely: maybe you're in a simulation such that if you act prosocially, your preferences will be rewarded.

Also as my decision-theory intuitions have improved I've come to appreciate this heuristic: when you can benefit others' preferences super-efficiently and so there would be massive gains from trade but you can't coordinate with your counterparty, just do your end of the trade anyway. (Even if you don't directly care about their preferences at all.) (Depending on context.)

(This is all imprecise/slippery; there's a few different meanings of "prosocial" and this depends on what kind of selfish rewards you care about.)

(h/t @Oscar for discussion.)

(I'm writing this to facilitate asking experts for takes.)