It's been done on several occassions. If you go to www.dauntlessmotors.com
and look under customers you will see at least one F650/sputnik. I think there are also a couple under "offroad sidecars"
If you google motocross sidecars or dual sport sidecars, you will find light platform sidecars. Never had one, but I am told that the true MX sidecars require a passenger or ballast as they are so light. They're like this: