I wonder how many coins they could get if they started doing this today.
I'm guessing a good chunk - 1/3 of all in existence at least. A lot of people would sell their coins before it was clear who was buying them. And then it would take a bit to realize the CB's intention. It would take some very high prices before everyone knew what was happening. Over $100k, I'd guess. We could hit $10k and nobody would even suspect it was any different than previous bubbles.
the risk for them doing this would be enormous.
reputational risk would include a market perception of fear of Bitcoin and lack of confidence in USD. it would be a clear signal that they believe their future is doomed. just "being in the game" would not facilitate monetary policy manipulation which is the raison d'etre for their existence in the first place.
if ppl found out they were doing this, everyone and their mother would pile in driving the price even higher to stratospheric levels. i suppose they could find a proxy buyer to accumulate coins but if they accidentally pick a Snowden-type, they'd be screwed.
CB's aren't smart investors as far as i'm concerned. look at Gordon Brown's faux pas. during the noughts, CB's in general were selling gold for the most part. the recent buying by Russia and China, i believe, will go down in history as yet another mistake.
no, i think the CB's, especially the Fed, are "all-in" with fiat. that's not good; for them.