On his blog "Understanding Society", Daniel Little has written an interesting post on "John Rawls and the EU".
Daniel Little's post is based on letters between John Rawls and Philippe Van Parijs in 1998. The letters were published in "Revue de philosophie économique/Review of Economic Philosophy" (vol. 7, 2003): "Three letters on The Law of Peoples and the European Union". See the letters here [pdf].
See also Glyn Morgan's paper on "John Rawls: Eurosceptic? European Integration as a Realistic Utopia" (2008).