I don't see a strong need for special symbols for currencies. Currencies can be represented using three letter codes like GBP, USD and BTC. It's ASCII, unambiguous and scales better than having a new custom symbol per character.
XXB maybe?
XBC is already used for "European Unit of Account 9". XBI, XBO, XBT and XBN are available.
http://www.currency-iso.org/dam/downloads/dl_iso_table_a1.xm...
For practical reasons, the ISO 3166/MA has "reserved the two-letter combination EU for the purpose of identifying the European Union within the framework of ISO 3166-1".