You have to keep in mind people are dumb and or do not believe what they are seeing. They think your wife gets off on flashing truckers but for some reason cant put 2 and 2 together that she is down to fuck. I say a simple paper sign and she gives them thumbs up or down and they signal back which one. Another option is to take her to a truck stop yourself in a slutty outfit and see who's interested. If they are she could tell them you want to join to film it, even hold their phone so they have a souvenir. I just can't see a trucker turning her down especially if she is giving it away for free.
Of course walking around a truck stop can have its own issues such as undercover cops but that should be rare.
Another problem you may run into, truckers now adays are very wary about people trying to rob them. some even take extreme measures to use their seatbelts to secure their doors while they sleep.
i agree letting her get plowed by random truck drivers is pretty hot, if you ever get to do It bring some plugs so she can deep their cum from dripping out.