Not really what you're asking for, but maybe it could provide the functionality you want without the weight of attaching a piece of plywood to your rack: Freerider pannier

I used to have a set of grocery bag panniers, and I got this to replace them. It's more functional and more versatile, I think. It's open topped, so obviously not waterproof, but it has a hole in the bottom to let water drain, so if you packed your stuff in dry bags, it'd work. Of course if you're main goal is to have everything laid out, so you can unpack from the bottom, that's not going to do it.

When I had a sleeping pad that was too big for a pannier, I rolled it up and attached it to the top of the rear rack, or, once or twice, tied it to the handlebars. I have sometimes had gear that's too bulky for a pannier, and in general I've found it more beneficial to get smaller, lighter gear than I have to find ways to carry bulky items.

All my tent and sleeping gear used to fit into one, Ortleib pannier, so it was easy to pull it off the bike and take to where ever camp needed to be. Although it was rare that camp was somewhere I couldn't just wheel my bike to. So while this sounds like an interesting solution if you routinely have to ditch your bike on the way to camp, it sounds like it might be more hassle than it's worth if that's not an issue you regularly face.

A pannier set with pockets can accomplish this as well, but not with the flexibility you're hoping for, I guess. I have now switched to a set-up that has multiple storage spaces rather than two, large panniers. The cookset is in the framebag along with water and tools. There's very little digging. Most items are separated into bags by use, and most bags are smaller than the average pannier. Essentially the bike becomes the backbone of the attaching system, rather than one, plywood pannier back. I don't have to go digging for an item, because there's only one bag that has clothing, and that's all it has. One bag for bedding. One bag for tent stuff. One place where the tools and cookware live. The only thing is that my Anything cages in particular lack that easy-on/easy-off functionality. that's why I try and use them for items that only need when setting up camp, rather than anything that I might want to access throughout the day.

That's why I like the Freerider. It doesn't have the ability to pull stuff out of the bottom or the middle that seem to be looking for, but it holds awkward loads while camping or while shopping, or it's a normal, old, grocery bag holder. My most common uses are carrying a cooler to the campground or carrying a backpack or satchel when I want to leave the bike somewhere and continue on with my stuff. And that might work, too, for you if you found a non-bike pack that had the kind of attachment system you wanted. Throw it an open topped pannier and pull it out when you need it. Doesn't help with water proofness, though. Good luck. I look forward to seeing what you come up with.