First, how possible is it for a country like the US to accept a decentralised asset as a reserve?
not 'as a reserve'.. but instead invest x% of the reserve in other assets.. US gov already invests in other assets they dont control, such as other nations bonds, other nations natural resources
You are right about the US government investing in other nations' bonds and even more, but we should not ignore the fact that these are centralised systems. They have the political power and economic rights for the countries to fulfil their obligations to them. This may not be a direct control but still the traditional way. Can we say the same about Bitcoin? The US will continue to tread carefully.
Again, the x% reserve you mentioned for another asset, is it not the same as depleting their reserve to buy another asset they will keep? Is that not reserving through another asset (Bitcoin)?