The only thing I could think of for a ‘messy’ look which won’t melt in the heat would be royal icing? It will set as solid, if not more so than fondant and you can get the unfinished look quite simply be spreading it on roughly. As for the chocolate cake, you could ask for it to be kept in the fridge of the venue and presented at the time of the cake cutting? I did that recently with the groom’s cake at the venue and the hotel staff were happy to make space in their larder.
