Do we actually want joint EU-US espionage 'working groups' meeting behind closed doors?

Yes, we want. The talks with EU would be necessarily more open and transparent than with national governments.

Americans are going to exploit few weak points national diplomacies could have (treat everyone special) and release there little information in exchange for compliancy.

Barack Obama isn't going to dispute in front of European Parliament (let's be real), but EU-US meeting, in style of UN Security Council, would be courtesy.