You're bound to draw attention to yourself, like the guy I called WDFW on today casting a bait rod on the Snoqualmie right above the FC bridge. If you have to have moving water, best to call ahead and let the WDFW know your intentions, so if a call does come in, they're aware of your presence and intentions. Take Aaron's Saturday morning casting lessons - they know he's there doing it.
It's just one of those things where you are risking getting hassled. You can mitigate that somewhat by informing the WDFW that you are going to be out there casting or going somewhere that you are unlikely to run into anyone. But that still leaves you open to random law-enforcement interactions and/or nosy do-gooders. Will you get a ticket? Probably not if you really are casting with a clipped hook and don't have any real flies on your person. But to be honest, it probably depends on the mood of the officer you run into and your attitude when interacting with them.
Isn't the whole occupy Skagit crew casting with hookless flies at the event. I'd think that tying a piece of yarn to show that you are obviously practice casting would work, but people would just think you were a dirty nympher from afar or maybe a yarnie purist
There is also the unfortunate side effect of others seeing you not knowing you are hookless and suddenly thinking the river is open for them as well. I know that's on them, but sometimes regardless of your intentions others copy inappropriately.
IF you do it, you better not have ANY viable hooks with you.
You've probably heard the joke about the woman getting a ticket from WDFW officer. She wasn't actively fishing but there was a tacklebox in the boat so the officer reasoned she had all the necessary equipment. She counters with "OK then, I am suing you for attempted rape". He sez "but why". "Because you have all the necessary equipment with you". "Have a good day ma'am".
No, it's not legal to fish with hookless flies if the water is closed to fishing. However, you can practice your casting with hookless flies in closed waters, but it's advisable to notify WDFW LE ahead of time to avoid being hassled by them and cited for causing them to waste their time on you.