It seems that "ex ante" views (like ex ante prioritarianism) haven't been discussed much within the EA community. Basically, the approach is to aggregate the utility in each individual first, over their life and by taking the expectation, and then apply whatever social welfare function you like to the resulting individually aggregated utilities.
Furthermore, you could take these individual aggregations/expectations conditional on existence (past, current or future), and only include the terms for actual (past, current or future) individuals; so the set of individuals to aggregate over would be a random variable. You'd then take another expectation, this time of of the social welfare function applied to these aggregated utilities over the set of existing individuals.
The main benefit here is to avoid objections of overriding individual interests while still being prioritarian or negative-leaning, since we can treat personal and interpersonal tradeoffs differently.
We define to be the aggregated utility of individual over all time (or just the future), in a given determined outcome (no expectations applied yet); in the outcomes in which they haven't existed and won't exist, is left undefined. Then we define
and we apply our social welfare function to the set
E.g., for some function which is increasing (or non-decreasing) and concave. Some examples here. Total utilitarianism has for all , and the ex ante view applied to it actually makes no difference. A fairly strong form of negative utilitarianism could be defined by for all , i.e. if and , otherwise; this means that as long as an individual is expected to have a good life (net positive value), what happens to them doesn't matter, or could be lexically dominated by concerns for those expected to have negative lives (i.e. only if we can't improve any negative lives, can we look to improving positive ones).
Finally, we rank decisions based on the expectation of over :
We can be both prioritarian or negative-leaning and avoid overriding individual interests; we don't give greater weight to the bad over the good in any individual's life, but we give greater weight to bad lives over good lives. Personal and interpersonal tradeoffs would be treated differently. You would be permitted, under an ex ante prioritarian or negative-leaning view, to choose great suffering together with great bliss or risk great suffering for great bliss all within one individual, but you can't impose great suffering on one individual to give great bliss to another (depending on the exact form of the social welfare function).
Let's look at an illustrative example where the ex ante view disagrees with the usual ("ex post") one, taken from "Prioritarianism and the Separateness of Persons" by Michael Otsuka (2012):
Two-person case with risk and inversely correlated outcomes: There are two people, each of whom you know will develop either the very severe or the slight impairment and each of whom has an equal chance of developing either impairment. You also know that their risks are inversely correlated: i.e., whenever one of them would suffer the very severe impairment, then the other would suffer the slight impairment. You can either supply both with a treatment that will surely improve a recipient's situation if and only if he turns out to suffer the very severe impairment or supply both with a treatment that will surely improve a recipient's situation if and only if he turns out to suffer the slight impairment. An effective treatment for the slight impairment would provide a somewhat greater increase in utility than would an effective treatment for the very severe impairment.
An ex ante prioritarian would choose to treat the slight impairment, while the usual (ex post) prioritarian who does not first aggregate or take expectations over the individual would choose to treat the very severe impairment. From the point of view of each individual, treating the slight impairment would be preferable.
EDIT: Here's an example which might seem pretty weird to some but also a bit intuitive to others:
Suppose there are two people who are equally well off, and you are considering benefitting exactly one of them by a fixed given amount (the amount of benefit would be the same regardless of who receives it).
Then, if you are an ex ante prioritarian, it would be better to choose one to benefit at random than to use a deterministic rule to choose one. However, the actual outcome will be the same, up to swapping the two individuals' utilities.
For what it's worth, under empty individualism (the view that one physical person over time should really be treated as a sequence of distinct individuals from moment to moment, person-moments), applying this ex ante modification actually doesn't make any difference. It'll look like we're overriding preferences, but under empty individualism, there are only interpersonal tradeoffs, no personal tradeoffs. See also "Does Negative Utilitarianism Override Individual Preferences?" by Brian Tomasik.
References and other reading
"Prioritarianism and the Separateness of Persons" by Michael Otsuka (2012) describes this approach, and gives examples to raise objections to prioritarianism and ex ante prioritarianism.
"Decide As You Would With Full Information! An Argument Against Ex Ante Pareto" by Marc Fleurbaey and Alex Voorhoeve (2013) cited in "Prioritairanism and the Separateness of Persons", has more criticism of ex ante views.
Toby Ord's objections to prioritarianism and negative utilitarianism which do not apply to the ex ante view: