I found a picture of how I did this! You can see the firmer, black foam with foil that I used on the outside, and the bits of softer foam that I used to pack everything in. The flap poking up covers the top when the pannier is closed. This photo is from when I was putting it together, so the contents is not well arranged. I make a simple frame so I could use the pannier as a handlebar bag so it got the benefit of the suspension fork.
I started out with an Ortlieb front roller, which is a smallish squareish pannier. I had a 300D + 70-300/5.6mm at that stage, so it was also fairly small. I got a sheet of foam about 30mm thick and carefully cut and folded it to make a box that just fit into the pannier. I added an extra layer at the bottom, since that's where most of the shocks come from. Then I cut more bits of foam to nest the camera and lenses. Basically set it up the way you'd set up any other camera bag, and arranged so that opening the pannier let me lift the camera out eaasily. Obviously this filled the whole pannier, so on tour I ended up with two full size rear panniers plus that front one. But the camera survived 10,000km or so of riding over several years.
With a larger camera you will need a larger pannier to start with, or sacrifice some of the padding. I suggest getting a large pannier, and making a padded camera bag out of foam to occupy the top half of the pannier, then making a cloth bag to hold that so you can lift the whole camera out easily to access the rest of your stuff. I'd probably keep my d lock on top of the camera round town so it's easy to get to.
The foam I used was house insulation foam with foil on one side. That worked well to stiffen it and make it more wear resilient - otherwise getting the foam box out of the pannier would have inevitably worn away the foam (I used the pannier for other things too). So a cloth cover would do the same thing, and should be easy enough to make as it doesn't need to exactly match the shape of the foam.
If you're looking for a commercial solution I doubt you'll have much luck - I've never seen anything. The best would be a small-ish camera backpack to hold most of the camera gear, then a pannier with everything else in it. That gets most of the weight off your back, but the expensive bits still benefit from the suspension effect of your body.
What I use now is a Crumpler camera side-bag that looks a lot like their bike courier bags but has a fuzzy interior that catches velcro, so I can use padded camera bag dividers to slot camera gear into it. It was easier to add the courier strap to a camera bag than convert a couier bag to camera gear (but it's not a great courier bag if you're on and off the bike all day).
For touring I have a custom recumbent bike with a special bin behind the seat to hold my camera backpack. I have a Desley bag that holds a laptop or a 1D body, but it's not quite high enough to hold both easily. It does, however, max out the cabin luggage size on most airlines. For getting onto the plane I usually put the camera around my neck, often with a big lens on it to bring the bag under the weight limit).