Hypothetically speaking, if I have a whole bunch of onions that I powdered, what value might that powder have as a fertilizer or soil amendment for plants like tomatoes, peppers and cucurbits (just burying a few to several spoonfuls in the soil at transplanting time)? If you know the NPK values for the powder, that would also be very helpful. Also, levels of the following nutrients would be nice, if you know them: calcium, magnesium, manganese, iron, zinc, copper, silica and sulfur.
No value at all, frankly, particularly not in the way you suggest using the powder, regardless of any possible nutrient value. I'm not 100% sure what you mean when you say you 'powdered' them, that sort of suggests you ground them up and dried them.
Onions are acidic as they decompose, and drive away worms and other bio organisms in the ground, which would reduce fertility in the immediate area. They're not even great on an open compost heap, though the odd chopped up few here and there won't hurt, but an absolute no no in vermicomposting. In their fresh state, they contain reasonable levels of magnesium, phosphorus, calcium, and higher levels of potassium, traces only of other elements such as selenium, not sure what happens when they're dried or powdered. It's a bit late to say it now, but you'd have been best off chopping the onions up and freezing them for use in cooking whenever you needed them.