If I understand the orthogonality thesis properly then it is possible to have any utility function. So if we were to try to satisfy the nonpersonal preferences of nonexistant people we would be paralyzed with indecision, because for every nonexistant creature with a utility function saying "Do X" there would be another nonexistant creature with a utility function saying "Don't do X."
This also means that your suggestion to "get some reasonable measure across the preferences of never-existent people, and see if there's anything that sticks out from the mass" probably wouldn't work. For everything that stuck out there would be another thing that stuck out saying to do the opposite.
Now Robin Hanson, who I think was responsible for starting this whole line of thought, replied to similar objections by suggesting that maybe not all non-existant people's preferences should count morally. But if I recall, one of his main arguments for taking nonexistant people's preferences into account in the first place was that if you started ignoring people's preferences, where would you stop?
So overall, I think that regarding the preferences of people who don't exist, and never will exist, as morally relevant, isn't a good idea.
Some people see never-existed people as moral agents, and claim that we can talk about their preferences. Generally this means their personal preference in existing versus non-existing. Formulations such "it is better for someone to have existed than not" reflect this way of thinking.
But if the preferences of never-existed are relevant, then their non-personal perferences are also relevant. Do they perfer a blue world or a pink one? Would they want us to change our political systems? Would they want us to not bring into existence some never-existent people they don't like?
It seems that those who are advocating bringing never-existent people into being in order to satisfy those people's preferences should be focusing their attention on their non-personal preferences instead. After all, we can only bring into being so many trillions of trillions of trillions; but there is no theoretical limit to the number of never-existent people whose non-personal preferences we can satisfy. Just get some reasonable measure across the preferences of never-existent people, and see if there's anything that sticks out from the mass.