I tour with a BMW four-cylinder machine, so the vibes are higher frequency, lower amplitude than your hog. That said, I got a purpose-built tank bag and bought and trimmed a solid lump of medium density foam rubber to completely fill the bag then trimmed appropriate pockets out of that with a sharp knife. The camera and lenses were a neat fit in the pockets, so to allow the items to be removed and replaced easily I lines the pockets by gluing in some satin cloth.
That combo survived several years of use for a Canon 35mm manual-focus camera and accessories.
More extensive padding than I do, but I agree, I would think some location between the axles is better. At least that was the theory I've operated under.
The earlier mention of bicycle seats has me interested, though... I'd have to check, but maybe the slip-on gel seat covers would fit various camera models well enough...
Yep, that's what I had in mind, couldn't remember bicycle seat though. The pad looks like it's got a drawstring too. Made for cameras. Get some wholesale call it camera pocket and triple the price. You'll be rich in no time.
Keep on gellin'