What do you plan on doing with the camera during this trip? How does protection need to be different? In the past, when you have carried it, do you feel it was adequately protected?
What I mean is, there is likely to be a greater chance you will run into bad weather if you are out longer. For shorter trips, there is less likelihood of a chance. However, when that occurrence occurs it can be just as bad weather whether you are on a day trip or or whatever.
What that means is, if you have felt protection was adequate in the past, why not now? If you feel protection is not adequate now because you will be out longer, you were taking chances with that occurrence occurring during short trips ;^)
I also carry several pounds of camera stuff. My camera is sealed. The kinds of lenses I prefer are not. Camera in holster, holster in mostly empty Talon. Because I worry, I keep a garbage bag handy in the top pocket.
I presume your worry is with the bike trip and not day walking with the camera. If the panier leaking is the problem, I might consider looking at what will make the panier mostly water proof and go from there. It wouldn't need to be a dry bag, would it? A plastic line you cal roll should do; the lid would secure it. Mmm, the panier is an enclosed thing, isn't it..
Where I worked in the past, we got super thick plastic bags full of parts. I kept some for diving gear to keep the trunk dry. Maybe there are some really heavy plastic bags for you to line the panier with?
This place has similar things, but they cost too much:
http://www.interplas.com/gusseted-poly-bags/3-mil-loose-pack-gusset-plastic-bags
You do get 500-100 of them, though.
As far as dSLR in ziplocs or whatever, I think that is fine to keep them dry. If you put wet things in a sealed plastic bag, water vapor will form, depending on temp, of course. I don't know if that will be a concern or not but it is something I would try to minimize.

