I just made lemon Mousseline with 1.5 oz fresh lemon juice, 3 oz Limoncello, and lemon oil to taste. It is quite good, though a little different than the variation made with lemon curd, which is superb.
The thing about mousseline is that it always seems to need liqueur to thin it out and make the texture soft and super-creamy. I haven’t made a caramel variation, but you might start by thinking about which liqueur you could pair it with - dissolving caramelized sugar in rum or cognac would be my first choice, because you have to be careful about adding too much sugar. I have also sucessfully reduced the amount of sugar added to the whites from 1/4 cup down to 1/8 cup, this should help keep it from becoming too sweet.
What are the other components of the cake? Would they work with rum or cognac?
Good luck and let us know how it comes out!