That scheme would fail due to basic forces of supply and demand. The government in this case would be a de facto market maker and the price would naturally get

That scheme would fail due to basic forces of supply and demand. The government in this case would be a de facto market maker and the price would naturally get pushed up to the maximum amount the government is willing to spend as speculators continually test the limit, which if the government has an 'acquire all bitcoins at any cost' mission, means the limits of its overall solvency. It's actually a similar scenario to what's currently going on in the bond markets. The Fed steps in any time bonds have a weak auction. Traders, knowing this, buy lots of bonds. It's basically a free roll for them because they know the Fed will always step in to prevent a major dip in price--and this cycle will continue until the Fed itself (aka, the dollar) collapses. So who knows, maybe they are dumb enough to try! :)