Outdoor wedding - need help dealing with hot weather

I am making a wedding cake for next weekend, but the wedding is going to be outdoors in a park and the forecast is for around 90 degrees and humid. There is no ability to refrigerate the cake after delivery except possibly in the caterer's cooler - but it might not fit. They want a vanilla cake with passion fruit filling. I usually cover my cakes with SMBC and fondant, but I don't think I can use the SMBC with all butter because it will melt. Has anyone made SMBC or IMBC with mostly hi-ratio shortening, and if so, how does it taste?

I don't love the taste of American (powdered sugar) frostings. I thought about using white chocolate ganache but I don't know if that would melt too, although the flavor would be nice with the passion fruit. Can someone with warm weather experience please give some advice? I never had to deliver a cake to a reception without access to refrigeration and I don't want the cake to melt in the hour or possibly two before they eat it.

Thanks!

Thanks for your response; I do understand that I provide the guidance to the client. However the forecast had been cooler and was revised today and now looks like it will be warmer on the wedding date. As a result what I am asking for advice about is not food safety (that I understand - time and temperature abuse, etc.) but rather what - if anything - other than a shortening-based American buttercream might I be able to use on the cake that would be relatively stable in warm weather? I know worst case I could use something like IndyDebi's frosting - I know many people love it. (No offense intended to IndyDebi intended by calling her recipe my worst case solution). But for me, no shortening based frosting, even IndyDebi's or one made with hi ratio shortening, can hold a candle to a meringue buttercream made with real butter or even to a ganache.

I haven't had the experience of making a cake that might be in such a warm environment for any period of time. This is a tough time of year to have an ourdoor event, let alone one without a kitchen. At this point I am contemplating lining a box with styrofoam sheets and buying dry ice to put in there along with the cake until it is time to cut the cake (I.e., a makeshift fridge).. But that doesn't change the melting point of butter so I to figure out what I will use instead of SMBC. What's the best alternative?

Thanks for any help you can give.

Thanks BakingIrene. That's an interesting idea with the pudding mix. I hate having to change all my recipes to try to figure out how to make this work. I think I'll have to do the shortening based frosting; I have hi-ratio that I will use. I'll experiment making a keep-cool box this week. This is turning into quite a project for a cake that was going to be pretty easy to do otherwise...