Best to just be honest and lay all of your cards on the table.
If you try to pull a fastie on them, you're just giving them an excuse to leave you twisting in the wind.
There is no "magic" that stops people from suing you- we live in a VERY litigious society- you can sue anyone, at any time, for any reason. And if there's no VALID insurance policy to go after, they'll just take your house and/or every penny you ever make for the rest of your life. The liability insurance on your vehicle serves a very important function, as a "buffer" against you sustaining these types of financial trauma directly.
But if they think that they're insuring a large, friendly Suzuki sport-touring motorcycle, and their fraud investigator's investigation reveals that you were actually using a land-based SCUD missle (with handlebars) that you assembled in your car-port over a few hundred beers, they'll RIGHTLY tell you to go pound sand up your butt, and walk away from you, leaving you with your pants around your ankles in front of people who are very well-equipped, very motivated, and very justified in doing you financial harm.
It could even become a criminal matter- want to become someone's girlfriend in jail over something like this?
Why would you lie to the people that you're paying to protect you from that? They're on your side, and you want to hose them? To save a few bucks a month on premiums?
The whole reason that insurance is expensive to start with, is because the insurance companies pass on the expense of insurance fraud to the rest of us- so you're really trying to screw us when you're dishonest with your insurance company. Every time one of us writes a check to our insurance company, we do so knowing that a good portion of that goes to cover the expenses of thieving sacks of shit committing insurance fraud.