If you have a good crusting BC, it should be ok. My worst weather one was in Richmond (on the Ind/Ohio state line) in August, 90 degree heat, a super high humidity (the thunderstorm hit about 15 minutes after I had served the cake, packed up and left). The cake sat outside for 5 hours (I set up at 2:00 p.m. ..... cake served about 7:00.)
no meltdown at all.
You might try the "High Humidity Buttercream" recipe.
http://forum.cakecentral.com/cake-decorating-ftopic-reply-374733.html
Is the bride willing to consider fondant? You could get a sample from Satin Ice for her to try.
I have one next month (July in Alabama, talk about HOT) and the bride thought she wanted fondant but talked herself out of it after buying some Wilton premade just to taste it. Sweetened chalk was how she described it. Oh no, no, honey, try this. *offer taste of Satin Ice chocolate, which tastes like fudge* All better!
I do have to say I have free rein of the cake so I could do fondant......but I have only tried the Wilton stuff and was kinda of curious of why anyone would put that stuff on anything edible....lol I guess I will have to try some of the recipes her on CC. But until then I will have to just keep it cold and use the crusting BC recipe that I have and hope for the best. Thank You all for your replies, greatly appreciated!!! 
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