What kind of console do they have? If it's analog and there's a side rack, I think a rack mount pre like the one Jon suggested would be ideal. Provided of course, they let you install it. IMO it's not a good idea to have some loose piece of gear by the board as it'll get lost/stolen/forgotten at some point. (Can't think of how many headphones, adapters, patch cables, CDs etc I've "lost"...)
On the other hand, if there's a digital board, you could run the mics straight through it and record from a matrix out. That would also allow you to create on-the-fly matrices, if the board is capable of delaying multiple channels/groups.