My 2 cents:
A butter cake needs to be at room temperature so it can be enjoyed. For a large wedding cake, I would have mine out for several hours prior to serving, otherwise the flavor is lost if the cake is cold. (Be aware, I live in a hot climate and we use lots of refrigeration). I would be reluctant to use a 'custard' filling with a butter cake, for two reasons. To keep it firm enough to cut, you'd need to use enough gelatin to make it unpalatable. The other reason, is the possibility of food contamination, since the custard would be at room temperature too long, since the cake needs to be room temp.
If you want to do a custard filling, try a chiffon sponge instead, and moisten it lightly with syrup, as too much syrup will make it gummy. Chiffon can be served at room temp. or chilled and it comes out great. I agree with Brown as to 'lightening' the custard with whipped cream and gelatin. You will also definitely need cake supports. I used "The Cake Bible's" suggestion of drinking straws and cutting them to the height of each layer.