Never heard of anyone using allium, basil, or oregano seeds.

I would rather save them to start new plants plus I can't say for certain that they are safe. Not all parts of a plant have the same chemical constituents as all the rest of the plant. Cherry pits, for example, are poisonous yet the cherries are incredibly edible!

FataMorgana

I can attest to the edibility of chive flowers! They have the same flavor as the foliage, only milder. Pick them when small, use them as garnish, in a salad, in an herb butter, in a cream cheese spread, etc. Use them whole or break them into the little florets.

None of these would actually be harmful, so I'd just try them out, if I were you. I often use chive flowers and would happily use the seed heads if I could be bothered to remove the papery bracts. I suspect that they would disintegrate into the food and be unpleasant to chew on if left on the seed heads. I have certainly used oregano sprigs including flower and seed heads in cooking but remove them at the end like bayleaves. My basil never gets to flowering point.