As of Dec 2006 there was a total of $820B in FRNs in circulation.
A report in 1995 concluded that 55 to 70% of all currency was located abroad and that the fraction was increasing. In 2001, the Fed estimated 50%. The Fed now says that the "majority" of currency is located abroad. If we estimate 65%, that would be $533B.
And THAT, my friend, is exactly how wars get started. Bad, bad idea.
Rather than being so heavy-handed, the US will just continue to debase the currency, so that investors get paid back with a fraction of what they loaned in terms of purchasing power. (BTW, that's another reason the government lies about inflation: if foreign investors knew how bad it really was, they wouldn't buy as many treasury securities and interest rates would be much higher).
All US debt held by foreign investors is held digitally. When someone holds US debt, they own a treasury security of some kind (bills/notes/bonds).