I had a similar thought for my pond, won't work on as high a scale as we would have liked. They will catch a few here and there but the plant's them selves take such specific care and eat so little that it would take an entire large bog type set up and remember only distilled water is allowed to be used to water on top of the expensive substrate they need.... just was not worth it for us. Of course if you all ready have a huge collection of the plant they that would save you some cash on the project too....
I don't think Sundew or Venus traps get large enough to handle all of the flies a coop attracts. You could try a larger species of carnivorous plant (i.e. Cobra Lilly, Pitcher Plant, etc.) but you may have difficulty with the husbandry. Are there any local species you could try?
My best advice when wanting to grow a new speciese of plant is to google it. I get so much detail from google it's insane! I had an idea to plan a few venus fly trap's maybe some pitcher plant's as well around my koi pond, then I realized id have to build a bog for the plant's first, and water with distilled water only, and plant in a special substrate (mainly peat moss) they have to be in a specific amount of sunlighe ect. ect. It's easier to get ducks, hang fly traps, or whatever else people do. Plus they don't rally catch that many fly's at once, you would need a small field of carnivorus plant's to keep up with a small coop....
I've grown and cultivated carnivorous plants for years, and I'll tell you, they are the PICKIEST plants. They have to have the right amount of sun, the right soil, the right temperatures, the right humidity, the right water. I have to to grow mine in temperature and humidity controlled aquariums and give them specially treated water because regular tap water will kill them.
They also honestly don't eat as much as people think they do. A venous fly trap will simply open again if it doesn't need the fly and release it.
There is no way you could grow or keep enough to actually manage a fly problem.
Though for anybody who loves challenging plants, I heartily recommend growing them for pleasure! They are extremely fun and beautiful. My handle "Ibicella" is short for "Ibicella Lutae" which is the scientific name for a semi-carnivorous southwestern plant called the "Devil's Claw."