I have a padded Lowepro waist bag that just takes my DSLR and one spare lens, that happens to fit in the tank bag. Have used that on at least one trip where all my luggage is used for other stuff.
When not using all my luggage usually use a Lowepro padded rucksack in my top box.
Although I don't really like doing it have once or twice just worn the rucksack to carry the DSLR (which I have also done while snowboarding/skiing)
I haven't come off the bike while carrying the DSLR on the bike in any of those locations to know how well it would survive, I have crashed while snowboarding (at maybe 15-20mph) with the rucksack on and slid along on the rucksack with no damage to the camera or lenses.
I have also seen metal edged, re-inforced equipment cases (that contain internal padding) strapped to bikes instead of top boxes.