What does 倒 mean in 现在也倒没有透露我们到底为哪个峰会准备这个午宴?
倒 is such a tricky word! I don't think(at least from I know) we have an English equivalent for it. The closest translation to English I think is "Well, (that one is different) ..."

Think about this dialogue: 


  A: 可以吸烟吗？    
  
  B: 不可以。
  
  A: 可以喝酒吗？
  
  B: 不可以 
  
  A: 那吃东西也不行了？
  
  B: 这个倒没有限制。(Well, that might be ok.)


As you can see, 倒没有 is usually used, when a serial of questions has been asked/talked about, and at some point the questioner asks the very question which he would expect the same kind of answer but the replier knows it would be a bit surprise(to the questioner) because his answer would be reverse(倒) from those answers he had given previously. 

For this case, 


  现在我们也倒没有透露我们到底为哪个峰会准备这个午宴。


There could be some other things that have been discussed in the previous context. After they have confirmed (or answered yes to) those things, and when comes to this one, they reversed the tone(倒没有) because they want to give a heads-up that this answer would be different from those previous ones. 

So, I would think 倒 in this context is expressing the idea "heads-up! the tone/answer is reversed". Or compared with the previous answers, this one is reversed(倒过来了).

The more tricky part is 也 in the sentence, implying there could be another previous question which has a reversed answer already. So, this one is the second one and that's why 也 is being used. 

Updates after thinking a bit deeper(which I think this question deserves):

This word 倒 is used when the speaker feel it's necessary to reverse the tone. This could be because of following reasons:


the tone is consistent for the preceding questions and at some point it changes. This is like the virtual example above. 
the speaker feels his answer would be the exact reversed from what his listeners expect. The OP's example could fall into this reason. 现在也倒没有透露我们到底为哪个峰会准备这个午宴。It implies that the speakers feel that normally people would think they've already revealed 我们到底为哪个峰会准备这个午宴, but reversely they have not. This reason could just be the speaker's assumption(Maybe, it doesn't matter if it's true). 
It could be a matter of idiolect. Some would use it casually without thinking that deep of the usage, especially in spoken.