Here in Portland you cannot put any structure or buoys in the water (such as slalom course buoys) unless they are for temporary use and only for an official sanctioned tournament. If you can prove it's for a sanctioned tournament and give them a set amount of days, they will clear you to put stuff like that up.
Probably similar wherever you are.
Like it's been said, it's one of those things where you don't ask for permission, you ask for forgiveness and be prepared to pack your stuff up, eat the costs, and possibly pay a fine if they aren't that forgiving.
Look at all the wakeskating and wakeboarding winching videos. If they aren't hosted by an event, it is like a ninja move. You get your stuff in, get your runs in, and gtfo as soon as you see someone coming and hope to get away without getting busted. Sometimes you're lucky, sometimes you're not. If you're going to do it, go ahead, just be aware of the potential consequences (aka fines, citations, loss of time and money building the whole thing)
Why not build the rails out of materials that look like they're just "debris" (repurposed wood from CL) you're using, and not actually rails. That way you can say you're just tricking off of stuff that happened to be there? It might be cheaper too. And since it is probably only temporary, a lower cost and lower durability would make sense