You realize that the word 'intended' is another form of the word "intent', right?They said intent is irrelevant. It's about 'intended design'.
Engraving "Airgun Moderator" or "For Airgun Use Only" on a silencer is not going to dissuade the authorities from arresting and prosecuting someone found with something that looks like and operates like a silencer.
Similarly, discussions about what the designer intended the device to be, may be convincing in court, but it's not going to keep you out of court.
Along the same lines, carrying some paperwork around that effectively says: "This is not designed to be a silencer." is also not going to carry any weight with an LEO.
I don't know when people started thinking that they are entitled to hold a trial any time an LEO encounters them and tries to arrest them. It doesn't work like that. LEOs are not judges. They make arrests based on probable cause, they do not try cases. Having an item that looks and operates like a silencer without the proper NFA paperwork is going to be sufficient probable cause for an arrest and for prosecution--maybe even sufficient evidence conviction depending on how the case plays out.
If that kind of thing worked, anyone could print up their "paperwork" saying: "This is not a silencer.", mark the silencer with "For Airgun Use Only." and the NFA would be out of business. The only way to be prosecuted would be to be caught with it actually attached to a firearm.
If you are confident that your approach will work in court and you have the time and resources to deal with that kind of a "solution", then go for it. You probably aren't all that likely to be caught, unless something else draws attention to you. Maybe that's an acceptable risk in your opinion. Different people assess risk differently.