Sadly I have to disagree completely, and I must say I find it sad that you come with such allegations without any reference. I have found test results indicating roughly a 50:50 ratio of scopolamine and hyoscyamine in young Datura stramonium plants, both for the seeds and the other epigeal parts. In one test, the average ratio was 47.2 % scopolamine and 52.8 % hyoscyamine for green stems, 50.4 % scopolamine and 49.6 % hyoscyamine for purple stems, 51.3 % scopolamine and 48.7 % hyoscyamine for seeds. In other words, this test shows young plants that on avegare actually had a slightly higher scopolamine content in the seeds than the stems!
There seems to be no significant difference in the ratio of alkaloids in the epigeal parts (seeds, flowers, stems, leaves) of Datura stramonium, but there are a significant difference when compared to the roots. This is not surprising, as it is in the roots the biosynthesis takes place. It is, however, correct that the seeds is the part with the least variation in total alkaloid content.
The data are attached, and there you can find all the numbers (taken from different authors from different countries) I am talking about. At the end of the article, you can see several other test results from several other Datura stramonium plants. In these tests, the scopolamine content in Datura stramonium seeds varies from < 1 % to 62.3 %.
Therefore, only older plants will have the desired effects! Young plants will still have some effect, but not nearly as good as you want. This is also the case for other Daturas, every member of the family has higher scopolamine content when young. As an example, Datura ferox has up to 99 % scopolamine as young plants, but 75 % is typical for older plants.
I repeat myself again - use only older plants! Seeds from young plants typically have from 40 to 60 % scopolamine! This decreases over age, until reaching < 1 % to 10 % scopolamine.