You'd still need to declare the install, even if no bottle fitted. Quite how an insurer would choose to define that risk I don't know, but certainly if you're intending on having any of it on display I definitely wouldn't risk it. Plus, if you have a big accident and then a guy comes out to inspect the damage, they'll be very quick to invalidate your cover when they see you have a kit installed, even if there's no bottle. I suspect you'd have a hard time in court not sounding like an extra from a FnF film too, should you try and argue your case.
For me, that's not something small that you could get away with, like a lightened flywheel. It'll be fairly obvious what it is, even at a cursory glance.