This might work with blackwater fish.
I have found that the best riparium selections are plants that grow pretty fast with lots of foliage to fill in. Most carnivorous plants are pretty spindly--they don't need as much leaf area for making food--so it might be more difficult to get them to look good in a riparium planting. Nevertheless, this could be a nice way to keep a few carnivorous plants in a display with fish.
I suppose you would just want to fill the planter cup with sphagnum moss and maybe something else a little more dense, such as hydroton, to hold the plants in place.