I'm growing several seedlings of Oroblanco and plan to use Oroblanco as the female parent in several crosses. Here's my reasoning for that.
The reason Oroblanco is seedless is because it is triploid. That was the whole point why it was bred to be triploid. The fact that it is triploid though shouldn't affect the formation of nucellar seeds. Only the formation of zygotic seeds would be aborted. In other words, for Oroblanco to be so relatively seedless, it probably can't be very inclined towards nucellar seeds (if it was there would be a lot more seeds). So, by crossing Oroblanco with another variety, I would assume that a fraction of that majority of potential seeds that don't normally form will form. Although only a small fraction of the pollinations will result in viable zygotic seeds, it's also true that only a fraction of the potential seeds, that don't normally form because Oroblanco is triploid, are nucellar. So from a probability viewpoint, there are two competing sides. The pool of potential zygotic seeds that have a small probability of forming are much bigger than the pool of nucellar seeds.
I've read sources elsewhere that say for grapefruit, somewhere in the range of 70-90% of the seeds that form are nucellar, and Oroblanco is a grapefruit x pomelo cross. (Most pomelos are 100% zygotic, but not all varieties)
One of the possible reasons, I think, that triploids are never used in citrus crosses is because very little is known about them (or at least that information isn't commonly known). Citrus breeders may not be inclined to try using triploids as the parent if they have no idea what's going to happen, or what the science is.
So, I suspect it might be possible that attempting to use Oroblanco as the female parent in hybridization might lead to a higher percentage of zygotic seeds than using a regular grapefruit variety.
If we look back at that study by Ollitroult, and assume that all the resulting triploid seeds were nucellar, then perhaps the other 50% (roughly) of [diploid] seeds that will form will be zygotic hybrids. (In other words, the percentage of hybrid seed obtainable from Oroblanco might be 50%, whereas for a regular grapefruit it might only be 10%. It can be really difficult/impractical trying to breed citrus varieties when a very large percent of the seeds in the female fruit parent you're harvesting seeds from are nucellar)
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zygotic = result from sexual recombination of chromosomes
nucellar = genetically identical to fruit parent, no recombination of genes took place, oftentimes because the resulting sexual gamete cells inside the seed wasn't strong enough to grow, so the nucellar tissue surrounding the gamete inside of the seed outcompeted and took over instead in its place