The problem with adding any vents at the top or near the top of the pants is that without some inflow from the bottom they are nearly worthless. I had a pair of rain pants many years ago that I made, and I inserted a mesh "window" in the front where you would have a fly if you had a fly. It did basically nothing to reduce condensation. For hiking, you can have loose openings at the ankles, and then a vent at the top could really do something. But for cycling I suspect that would mean you'd get soaked on the lower legs, which would put you back at square one. But you could try it.