Interestingly, the law specifically includes "foreign currency" in the definition of monetary instrument.

No, it doesn't. It says any currency, along with all kinds of things that aren't currency but which can more-or-less readily be converted into currency, including travelers' cheques and negotiable instruments, both of which are probably less liquid than bitcoins. Indeed, a bitcoin is a "negotiable instrument".

I quote:

(5) the term “monetary instruments” means

(i) coin or currency of the United States or of any other country, travelers’ checks, personal checks, bank checks, and money orders, or

(ii) investment securities or negotiable instruments, in bearer form or otherwise in such form that title thereto passes upon delivery;