I'm confident that there are many miners already doing this, include me (maybe even some mining companies). And it does not incur any loss, that's the reason after each bitcoin hype, it crashed to a new higher low
For example, I have sold 10 coins with an average price of $800, received $8000. Now if bitcoin price crashed to $400, I need only $4000 to buy back all the coins I sold, thus there will be no net sell pressure from me and I still made $4000. I can even spend $6000 to buy 15 coins thus both of my bitcoin holding and fiat money holding increased after the crash
Sorry, but this doesn't work that way. If you sell something dear and then buy cheap, it is called speculation. If you sold some coins at $800 and received $8000 in return and bitcoin price then crashed to $400, it means that some poor wretch has lost $4000. I just can't fathom how this could possibly be called support for bitcoin price...
Why those poor guys lost $4000?
Because they want to sell the coin at a lower price, if they don't sell, I won't be able to buy coins at $400. So their loss purely comes from their lacking of confidence of their investment, if they never sell any coin, they won't have any loss, and I don't need to support the exchange rate either
From the view of the central bankers, they normally don't care about the loss or profit, those are trivial things comparing to their large holding of fiat money and foreign currency reserve, the exchange rate is the only concern
But this in no case means that you're "supporting" the value of bitcoin. If fact, I would even call such stance hypocritical to a degree, since you're in the first place undermining its value by selling coins at $800 and then pretending to support it by buying it at $400 . This is plain old speculation, nothing beyond that really...
If you only bought bitcoins when the price went down (and spent it on goods or services), that would go for support