I had asked a question before about whether white chocolate ganache tastes good with a lemon cake. Per the recommendations, we tried it and loved it.

Now I'm concerned that I can't do that after all. 

June wedding in Tennessee - reception in unairconditioned barn. The cake can be in an air conditioned area that is off the barn until right before the reception begins. The plan is to eat dinner and then do some dancing and then cut the cake. 

Would the ganache/fondant cake be okay for about 90 minutes out of the air conditioning during the reception or should I switch from ganache to a hi-ratio buttercream? 

This is a wedding cake for my daughter and I want to get it right. (I'm just a hobby baker but have lots of practice with cakes for family and friends.)

Hoping someone can help! 

If you really want to use ganache, just increase the chocolate to cream ratio - you can go up to about 4:1 in hot weather.

Maybe experiment to see what holds up in your climate - just crank the heater up at home to simulate summer!

What winniemog said, plus keep it out of the direct sunlight always.  White choc melts at a lower temperature than dark chocolate - if the barn is under 29C / 85F you should do fine for 2-3 hours after it leaves the nice air conditioning.  The fondant will act as a bit of an insulator for the heat.

Thank you very much!

It won't be hard to simulate that kind of hot weather here! It should be in the mid 80's pretty soon.