I planted a Bloomsweet close to a Dunstan citrumelo with the very intention of possibly making this cross in the future.
It will still be a long time until they grow big enough to fruit though.
The Bloomsweet is moderately close up against the house, on a south-facing brick wall, in a warm sunny spot that I thought would be most optimal to plant something that was going to be kind of marginal in this climate (Olympia, WA, zone 8a). So far the Bloomsweet is managing to survive.
I'm guessing probably around 55 to 70 percent of the seeds in Dunstan citrumelo are nucellar, meaning the percent of seedlings that could turn out to be hybrids is smaller.
The percent of nucellar seed could even turn out to be something more like 85 percent.
I just quickly looked up one reference that says the percent of zygotic seeds in Swingle citrumelo ranged from 9.3 to 17.7.
It could be higher for Dunstan citrumelo, but I believe Dunstan and Swingle originated as siblings, if I'm remembering correctly.
No idea what it is for Bloomsweet. The original parents of Bloomsweet are believed to be Kunenbo (which would likely be similar to Satsuma, at 90 percent nucellar) and pomelo, which are fully zygotic seedlings.
A variety with a very high percentage of nucellar seeds means it will be more difficult to use it as the female parent in making hybrid crosses, since most of the seeds will turn out to be genetic clones of the fruit parent, and you can't immediately tell which offspring seedlings are actual hybrids. In the absence of some identifying trait, at least. One common identifying trait that has often been used is if the offspring from a parent with normal leaves displays trifoliate leaf shape. Then they immediately discard all the seedlings with normal leaves, even though a portion of them could turn out to be hybrids as well.
I believe a Bloomsweet x Dunstan citrumelo cross could turn out to be very promising. A Dunstan citrumelo is not that far away from being edible (from what I've read), and I believe just one more cross is all it would take. The Bloomsweet is also supposed to be just a little bit insipid and could probably benefit from the stronger level of flavor from the Dunstan citrumelo.
Then, after making the first cross, I would grow a bunch of seeds from the hybrid and see if any turned out to have improved fruit quality from the first generation hybrid. Sometimes genes can get mixed around again, recessive genes can express themselves and certain dominant genes can get eliminated. (referring to sexual recombination from self-pollination)
That's what I would do.
The downside is, in these northerly climates which we are in, the time between a small plant and getting it to finally fruit can be very long.
I also want to say I'm very skeptical of a first generation Bloomsweet x citrumelo hybrid being able to survive in Germany zone 7. Bloomsweet is moderately hardy, but nowhere near zone 7 hardy. For that, I think you would have to grow several hybrids, and then from the fruits of each of those hybrids, grow many seedlings, and then you might finally see citrumelo-like cold hardiness again. Obviously more difficult than just simply creating a hybrid.
For the colder part of zone 8, and the very edge between zones 7 and 8, I think this would be a good idea.