I have a cake due this Saturday. I only just yesterday brainstormed compatible fillings for the cake flavors my client chose. 

The top tier is, what I call, Autumn Mixed Nut Cake. It has walnuts, almonds, pecans, and hazelnuts. It is fairly sweet so I thought that a Honeyed Mascarpone buttercream might be the perfect flavor profile to even it all out.

I always use an Italian Meringue Buttercream variation for my tiered cake fillings. It's reliable and sturdy enough for the projects. 

I'm concerned now that simply adding mascarpone to IMB will make it looser than I need it to be and also not taste enough of mascarpone with all that butter in there. 

I'm in a need of either a reliable Mascarpone IMB (not an American buttercream please - yuck) OR a fantastic Honeyed Mascarpone Cake Filling Recipe. HELP!!

you will keep this cake refrigerated all the time right?

try here:

https://www.google.com/search?q=honey+mascarpone+frosting&ie=&oe=

Yes. Absolutely