I've recently invested in a set of the pockets for my back-roller classics, in the interests of quarantining wet stuff more than the additional capacity.
Fitting them is scary, as getting them accurately positioned means making precise measurements on a uncooperative flexible surface. I recommend stuffing the pannier with something suitability amorphous, then using gaffer tape to play around with the position of the template and then marking the hole positions with something that shows up on Ortlieb fabric. Piercing the fabric with the supplied tool and attaching the mountings is straightforward, and I see no reason the end result is any less waterproof than the mounting of the pannier's hooks (the hole you make should be a tight fit for the self-tapping screw anyway, and the fixings surround it from both sides).
The pockets (and presumably bottle cage) attach in a quick-release fashion, by temporary bending of a plastic strut on the removable part to fit into slots in the mountings. It's simple with no moving parts, and is a lot more secure than that sounds. It might not stand up to large numbers of removal/attachment cycles, but I expect you can obtain spares.
Obviously it means you've got a thing sticking out of the side of your pannier. Having this on the front side is a non-issue on a recumbent (which is all I've used it with so far), but I expect it needs to be on the rear side of the pannier to avoid heel strike on an upright. In turn, that probably makes it vulnerable to being bashed when manhandling the bike for parking/trains/Silly Sustrans Gates™/etc. If used for a bottle, I can imagine it getting some mud-splatter - a bottle with a lid would seem prudent if you're going to be riding across fields of wet sheep shit.
Here's some quick-and-dirty photos of one on my Dawes Discovery, which IIRC is an 18" frame. Wheels are 700c. (Apologies for the atrocious white balance - trying out a new phone camera in crappy light):
Side view:
I've moved the hooks slightly further back on the rack for this one (they're not properly adjusted for this rack), but I doubt you'd avoid heel strike (also note this is a 160mm crank!):
Here's the fittings on the pannier:
And the back of the pocket:
ETA: For the avoidance of doubt, these pockets are available in two sizes. These are the *smaller* ones. The SJS website observes that while you can fit the large ones to a Back-Roller, the pocket will hang lower than the base of the pannier.