You're correct that there's nothing in the Companion on this, and it is entirely possible Brandon came up with the Seed idea, but he does seem to have left the door open for a manufacturing heavy, highly technical second stage to making angreal.
And while Brandon may entirely have come up with the Seed himself, it does seem to have some basis in the series mythology. Assuming RJ wrote that the Finns can drain your OP strength (which I think is true, since Lanfear lost her ability, as did Moiraine, in the notes), there's already some basis for this. For another, the making of the Eye of the World also hints at spirit-y soul-y stuff, not advanced manufacturing. RJ even had Elayne feel a personal bond with her "wise woman" angreal, and speculate that it was a personal angreal.
So if it is a Brandon creation, it is a much more clever one than some other stuff he came up with for WoT.
I agree it's more clever, but I think it's got nothing to do with what the Finns do, which I rather suspect is similar to what a gholam does to channelers, draining them until they die. I still believe Aginor used Finn DNA in the making of some Shadowspawn.
I didn't get the impression that BS was hinting at complex manufacturing: what would have been the point for Rand to even give a Seed to Elayne if that were the case and she would pretty much end up stuck at the end of the process. I rather got the impression that BS intended us to think Elayne would figure out the end process by studying and copying other angreal.
