I thought about it some more -- even the private lake/waterway is a no-go. You can't drive any vehicle without an IID, even on private property, if the vehicle is the type that COULD be used in public spaces. ORS 801.590 (“‘Vehicle’ means any device in, upon or by which any person or property is or may be transported or drawn upon a public highway and includes vehicles that are propelled or powered by any means".)
Thank you for your detailed explanation and citing text in your response. I appreciate your time and I'll enjoy watching the boat ramp shenanigans!