"It was very difficult to cross Ipomoea hederacea with Ipomoea purpurea. In 1980 the author collected a lot of strains of both species each and crossbred them on a large scale, but obtained only one seed after about 2000 crossings. From that single seed, the author was able to bring up an F1 plant, which set seeds. So F2 descendants were grown. "
http://protist.i.hosei.ac.jp/Asagao/Yoneda_DB/E/relatives/04_4.htmlSo, viable seeds or crosses may not happen just because one wants them to or has made careful pollinations.
Datura frequently self-pollinate so regardless of insect activity... hybrids are very unlikely, but they can happen. You can be relatively sure that your seeds are not hybrids however with Datura unless you have taken the time to emasculate and remove the anthers prior to the flower opening and applied the foreign pollen yourself at that same time.
Datura on the other hand has been crossed to Brugmansia, but the successful embryo rescue of plants did not result in self-fertile offspring. Datura metel var. chlorantha will readily hybridize with Datura metel var. fastuosa however and the resulting flowers are most often co-dominant in color.
Datura wrightii and Datura inoxia both make excellent pollen donors when bred to Datura metel and many seed pods may form. However, many pods may also abort and the number of seeds that make it to maturity within a pod are generally greatly decreased in my experience.
Datura stramonium will cross to Datura ferox.
Brugmansia are not self-fertile except for B. arborea as a general rule. There are a few exceptions when dealing with individual cultivars.
Brugmansia can be divided into two main groups. A warm group consisting of Brugmansia insignis, Brugmansia aurea, Brugmansia versicolor, and Brugmansia suaveolens that can all be bred together. Brugmansia candida = Brugmansia aurea x Brugmansia versicolor and such crosses can result in a double flowered specimen if one grows out enough seedlings. Always breed like colors to like unless you want hundreds of whites and a very few in color.
Brugmansia cold group... consists of Brugmansia flava, Brugmansia sanguinea, Brugmansia vulcanicola, Brugmansia arborea. Note; flava = Brugmansia sanguinea x Brugmansia arborea. I have only been able to get Brugmansia arborea cultivar "Ida" to grow and bloom profusely in Florida as most warm group Brugmansia simply perish soon after being planted in Florida. I am currently growing a single Brugmansia arborea from seed out of several thousand that has been able to withstand the heat and humidity where I am currently located as I lost "Ida" when I moved over seas.
I have been able to get seed pods on Datura X Brugmansia can often result in seed pods, but the seeds themselves will more often times than not abort when still very tiny and if you did intend on making this cross and actually growing them out you would need to rescue them long before the seeds start to shrivel and then grow on some kind of agar media. My understanding again is that all such crosses would not be self-fertile... so you would need to different specimens to ensure even a remote possibility of making seed and you may even have to induce polyploidy via colchicine or some other chemical if you wanted to proceed further.