Max, i built a "hoop trellis" from a steel frame for a hoop house (basically the same as the conduit) covered with "remesh" (concrete reinforcing wire). The remesh is very sturdy and definitely does rust, so that may be what you have seen. I don't think there is any adverse effect to the plants from the rust, although it isn't particularly good for clothing...
In 2014 I grew small gourds on my trellis
Since it sounds like you only need half of a hoop (the way I understand your post), or even if you want to do a full hoop, you might want to consider using "cattle panels." Here's a thread with some good info about using them The thread "Cattle Panel Trellis Arch"
in Garden Structures forum
Also, I don't think you will need support for heavier melons and such -- in 2015 I grew several types of squash and birdhouse gourds on my trellis, without any of them dropping off.