Newbie here, what a great place!

Here's my question: I have an order for a fondant cake with a cream filling. In the past I have stayed away from cream filling in fondant cakes, but my client REALLY wants this type. I have found that the pastry cream and bavarian cream fillings tend to get soft sitting out during the reception and can start to bulge out from under the weight of all of the fondant. I always use a ring of stiff buttercream to seal it in and I'm always careful to use lots of dowels.

Refrigerating up until the time of delivery isn't an option because this will be decorated with black RI on white fondant. If condensation forms on this cake it will look like a big mess of running mascara!

I've looked at a lot of cakes and notice that every once in a while other fondant cakes suffer from that same dreaded bulging midsection. Does anyone have a recipe they would recommend? Some magic technique to share?

Ive never had a problem with bulging , never. ive only used whipped cream once as a filling, i saved off a small amount of and whipped it till it was so stiff my arm couldnt take it anymore, then i used that for dam, and then added the lighter, fluffier cream to the middle. just made the dam a little extra strong ya know? but then agen, i dont put in a huge amount of filling because i normally work with thinner layers but hopefully some one else may have a good idea for you

There are two ways about it:
1.
refrigerate - the RI if you get the RI completely dry before you put it in the fridge, it shouldn't run. Put the cake in a box, the box in plastic bag, close well. Set the fridge on maximum cooling. To be extra careful and if your design allows it - pipe with white RI first and overpipe with black. I know, it is twice the work.
2.
Stabilize the whipped cream with gelatin.

Either way I would use the buttercream dam. Not to hold the shape, but because these types of filling shouldn't touch the fondant. This can make worse mess then weeping from the black RI.