Not only is there such a thing as a common system for international wire transfers in Europe (SEPA Credit Transfer, which is also used for domestic wires as well), instant payments are coming soon™ to that (supposedly end of next year): http://www.europeanpaymentscouncil.eu/index.cfm/sepa-instant...
The existence of SEPA shows that it might be viable on a larger scale. While the EU and EEA members already share a common legal framework, the other two SEPA members (San Marino and Monaco) don't (even though they cooperate closely with the EU in certain areas). With enough political will a larger payment area doesn't seem impossible. (But personally I can't imagine this to happen globally.)
