Not much. German nationalism was strong and getting stronger.
And France was the last power any German would listen to. Germany had been occupied and plundered by Napoleon's armies well within living memory [1]. Any French objections to unity could only rally German opinion in favour of it.
[1] King Wilhelm I had been a serving soldier in 1814. No doubt quite a few other Germans could have said the same.
I'm guessing France's best bet would have been to help Austria retain dominance within the German confederation, since their attention would always be split between Germany and the Balkans (and possibly Italy). After 1866 that was no longer workable, though.