My friend has asked me to make her wedding cake in August and would like a 4 tier red velvet cake. This will be my first 4 tier cake. I usually use a cream cheese icing as a filling then Swiss meringue buttercream on the outside for my red velvet cakes. The ceremony is in Brisbane, Australia and starts at 3:45 so I'll need to set the cake up in the marquee beforehand. It is still quite cool in August but I've been advised that they'll be heaters in the marquee so now I'm nervous about using cream cheese icing and Swiss meringue buttercream. Do you guys think the icing will hold up if it sits out for a few hours? Do you have any suggestions?

How hot do you expect it to be in the room?

I'm not sure. It will still be quite cool at night in August so I guess they'll have the heaters to make it more pleasant but I'm guessing not so hot that people are sweating. 

As long as the cake is not close to one of these heaters and the heat in the room doesn't get out of control, it should be okay... I would ask the bride for specific details about where the cake will be displayed in relation to the heaters. Hopefully it doesn't get too hot in the room; much hotter than room temp and I would start getting nervous. If necessary, maybe you could plan on having the cake taken away and refrigerated after the couple cuts into it? Do whatever you need to keep that cake safe! :)

They're planning on having it for dessert so it should be cut up straight after they cut it. Do you have any suggestions for a more stable buttercream? 

I don't have a lot of experience with SMBC (I've only played around with it a few times) so I can't be of much help there! I would google something like "how to make SMBC more stable" and I'm sure you'll be able to find something. Best of luck!