This is an interesting filling concept I haven't tried. Been playing with fondant and adding cooked down puree (mango, passion fruit, pineapple cooked to 104 with glucose) at 1/3 - 2/3 proportions which is decent, and using it for a take on a cream egg by piping a "yolk" with it into white fondant. When you make it into a buttercream with chocolate and butter, what kind of proportions do you typically use? Does it stay creamy? Should be super shelf stable I would assume.