I looked up some stuff about Argentina’s financial crisis since you mentioned it before, and it looks like they actually did something a bit like what I’m talking about, directly appropriating the valuable assets they could in an effort to keep being able to function:

In addition to the corralito, the Ministry of Economy dictated the pesificación; all bank accounts denominated in dollars would be converted to pesos at an official rate. Deposits would be converted at 1.40 ARS per dollar and debt was converted on 1 to 1 basis.[69]

There’s some indication that this also applied to financial products:

As noted above, a number of U.S. investors have filed ICSID arbitration claims against the government of Argentina. Most of these investors consider the January 2002 pesification of dollar-denominated contracts, and/or the ex post facto prohibition on contracts linked to foreign inflation indices, to be an effective expropriation of their investments

I can’t specifically confirm this included gold held on paper, but I think it probably would have.

As for the plausibility of this sort of thing happening in the US, in addition to the actions of Roosevelt mentioned by @diablexical@sh.itjust.works, the main trigger for Nixon abandoning the gold convertibility of US dollars was France attempting to physically withdraw the gold they had stored in US banks, which they didn’t want to allow.