I have a bonneville and love it. Last year I bought a Ural, and each time after a ride, my wife and I agree it is the best thing we ever bought. I commute to work each day on my Bonny, but occasionally take the Ural for a change of pace or if I'm carrying a lot of stuff. I spend most weekends riding the Ural over the Bonny. It is not a fast bike, but of course that's not why you get a Ural.
My cage is a Jeep Wrangler, if you can call it a cage. i only drive it if the weather is really lousy. Hacking the Bonneville? IDK, I'd consider it, but I do like two wheels, and riding fast. For the cost of a Jeep, even used, you can probably get another bike with a sidecar. There is the guy is New Jersey that sells hacked Bonnevilles for around $10k, I don't think there are too many used Jeeps worth buying for too much under $10k.
If I already had a car and had to choose "bike with sidecar" or "Jeep," mainly for fun, bike and sidecar wins every time.
Living the dream of the 90's