Pagnr, can you elaborate on the concern about open pollination not yielding the desired results?
My point is that I am in a Citrus growing area, and often collect Citrange, Swingle, ( also Rough Lemon ) fruit from seeds for rootstocks from various garden trees,
where the rootstock has taken over, or partly taken over, ( so there is a half rootstock half scion fruiting tree ).
As these trees are usually in gardens with other Citrus, there should be opportunity for random cross pollination.
I don't think I am seeing any significantly different seedlings to indicate this happens.
For commercial rootstock seed production, trees are in arboretums or rootstock tree seed blocks.
Again there should be ample opportunity for cross pollination.
From these rootstock seed trees, tens of thousands of Citrus rootstocks are grown.
There are issues of off type zygotic seedlings, more or less % with the various types of Citranges.
I don't think there is any issue with rootstock seed being out crossed to neighbouring trees.
Some Citrange types produce higher % of zygotic, but these don't seem to be outcrossed, they are off types of the same variety.
Zygotic seedlings do not necessarily need to be the result of outcrossing to another Citrus, they can be self crossed to the same variety, possibly the same flower.
There may be flowering period timing issues as to why the outcrossing is not seen, ie the flowering period of two adjacent varieties does not overlap enough.
Also there may be pollinator behaviour issues. Bees may not move between trees, but work the same tree and rarely hit another tree to X pollinate.
When seed saving heirloom vegetable varieties at risk of X pollination, there are strategies to reduce this by interplanting other species to buffer the pollen movement by bees.
For mixed rows of Fava broad beans, type A type B type C, if you only collect seed from the middle of each type, it is unlikely that bees have jumped between types along the same long row. Only the adjacent plants in the row are likely to be crossed. If you plant peas between the broad beans types in the row, the chance of X pollination of varieties is further reduced due to bee movement behaviour.
There may be ways to reverse this and optimise X pollination.
Secondly with random pollinations, you will have no idea where any successful seedlings have originated, parent wise.
You may be able to guess the parents from characteristics, but if you want to repeat a cross to get more versions of the same, you will have to start from scratch.
Another point to consider is the way you grow out the seeds.
If you mix all the seeds from all the fruit from one tree and grow them in one lot, you will see random odd seedlings.
If you grow the seed from each fruit separately in its own pot, you can track pollination events and related crossed seedlings.
ie if you grow a container of 100 citrange seeds from ten fruit and find 7 off types, you will have an off type rate of 7/100.
If you grow the seed of each fruit separately, you may find that 5 off types came from only one fruit, and are likely X pollinated ??
The other 2 off types may be both in lots from different fruit, and are likely zygotic.
Hopefully you can then distinguish the characteristics off off type zygotic vs X pollinated seedlings.
Initially you may not want to extensively X pollinate, as it is a lot of work.
You may be able to filter results by separating seed lots by fruit extraction.
You may find differences in % off types, vigour, disease resistance.
If you mix all seeds from all the fruit from all trees in one season, you will have random unrepeatable results.
The more you seperate the seed lots, the more you can see patterns.
Labelling is very important, it can be hard to keep track of what's what as they build up.