As for policing it, all it takes is for every N reg to be met for a couple of months and the crew if not able to produce a JAR license or a green card/US passport get thrown in the nick and the aircraft impounded until its all sorted out.
I don't think it's quite that simple; the rules are based on whether or not the operator is actually based in the EU, not the nationality of the crew as such. Pretty obvious in the case of someone stepping out of a privately owned Cessna who lives in the UK but for larger aircraft it might not be so clear were exactly the "operator" resides. How that will be enforced and whether or not people come up with elaborate ways to "base" their aircraft outside of Europe or just bite the bullet and comply remains to be seen...