royal icing and candy atop buttercream
I think you'll be fine. I had some buttercream frosting left over from answering a previous question and did a simple test not long after you asked this question (about 16 hours ago). 

I put a dish of frosting into the fridge with small royal icing and sugar/starch based decorations on top and I left a similar dish at room temperature. I pushed the decorations firmly into the frosting.

The decorations retained pretty much all their crunch. They didn't leak any of their bright coloring into the frosting either; not that that would be a problem if you're decorations are snowflakes. I'm not sure whether the opposite would be a problem, i.e. if your frosting is brightly colored and your decorations are white.

You can judge for yourself whether the ingredient proportions I tested with are comparable to yours by checking the answer I linked to above. You may see different results if the proportion of wet ingredients in your recipe is much larger.