Will a Swiss Meringue BC cake hold up in 75 to 85 degree weather or will it melt?

The recipe I use for SMB can substitute the butter for shortening.  I have never tried it, but you might do a trial run to see if it works.  Another option could be white chocolate ganache.
 

Heat will possibly make it very limp...and while there is an average, there could still be some higher than usual temps at that time of year.  

The bigger concern for using a SMBC and an outdoor wedding is that if you have a gust of wind or a dust devil come through, or even a stray gnat or fly...it is going to stick into the cake because there will be no crusting or crystalizing of the buttercream as with an american buttercream.  

When I have to do a buttercream cake that will be exposed to the elements (be it fully outdoors or in an open air area like on a roll-back door patio) and the customer wants the taste of SMBC but not fondant I suggest doing the fillings inside the cake in the SMBC but the outer frosting layer in an American buttercream that is both more heat stable and able to resist flying elements. Naturally, fondant is the best choice but ABC also gives some protection. 

Quote:

Originally Posted by lorieleann 

Heat will possibly make it very limp...and while there is an average, there could still be some higher than usual temps at that time of year.  

 

The bigger concern for using a SMBC and an outdoor wedding is that if you have a gust of wind or a dust devil come through, or even a stray gnat or fly...it is going to stick into the cake because there will be no crusting or crystalizing of the buttercream as with an american buttercream.  

 

When I have to do a buttercream cake that will be exposed to the elements (be it fully outdoors or in an open air area like on a roll-back door patio) and the customer wants the taste of SMBC but not fondant I suggest doing the fillings inside the cake in the SMBC but the outer frosting layer in an American buttercream that is both more heat stable and able to resist flying elements. Naturally, fondant is the best choice but ABC also gives some protection. 

That times 1000.  I did a cake this last year that was at a yurt on the cliffs overlooking the ocean.  The cake and dinner was going to be all Pinterest rustic and outdoors.  I tried to tell the client that the cake is going to be a giant sandcastle by the time they wanted to serve it but they were convinced by the yurt owner that the yurt would block the wind and protect the cake from any sand kicked up on the beach.

What they didn't realize is that the dirt road everyone had to take to get to the yurt was, in fact, DIRT.  I got an email that they loved the cake but wished they would have listened to me when I told them the cake should have been IN the yurt, not next to the yurt!

And yes, I had to Google "yurt" when I met with them.

To answer OP's question, if you deliver it completely refrigerated and it is at least under some sort of shade and it's and evening wedding, I would not be worried.  Average temperatures are generally the hottest parts of the day, which are noon-2.  If you deliver a totally refrigerated cake at, say, 4, the cake isn't in direct sunlight and it's under 78 degrees when you deliver, it should be fine.  

Thank you FromScratchSF!  I l learned a new word today (yurt) and will now have to use it in a sentence tomorrow.  LOL!  BTW, it still sounds neat, dirt and all.

I agree that it will be fine, as long as its out of direct sunlight. One caveat, avoid great festoons of buttercream or buttercream roses. they will probably lose their sharp edges as the night goes on.