Thanks, does the zip have a slider on it, or is that removed during manufacture?

Looking again at Elsawin/Bentley it seems as if, with that centre pad removed, you can get at some fixing wires for the side bolster cover to free that. Whether that is required will depend on exactly where the pump is.

The wiring and pipes (and fixing screw) are all "below" the pump (i.e. towards the seat hinge) and probably fairly accessible; so if I can force enough access without dismantling I might try to disconnect it and jury-rig it to the new pump for testing. If I totally disable it during that process I've not lost anything, but I'll start on the passenger seat anyway. ;)

I'll have a look tonight how the zip looks on the backrest pad.
Yes its true you may need to unhook one of the side bolsters to reach under it (after the backrest pad has been removed). The brown leather pouch may be covered by the bolster.
The bolster is only secured to the backrest metal frame by a long hooked wire, to which wire clips have been pressed.
On the first bolster I wasn't thinking and unwound all the individual steel clips, but on the second one all I did was unwound the hook of the long wire with some pliers, then pulled it through all the clips. This is definitely the way to go.

Looking again at the Nordic Audi photo that I linked to (in my thread) the pump unit is at the same level as the big hooks for the centre pad above the lumbar pads (with Velcro in your picture, but not theirs).
https://nordicaudi.com/php/files/896135-DSC00286.JPG
It is fixed to the metal back plate of the side bolster on the left side so fairly buried inside the bolster; it will indeed be just above that "brown leather" wrap.

I've just confirmed by operating the pump that it's on the left side on both front seats and I can get my fingers inside the bolster to just about reach it. However a rigid joint in the centre pad gets in the way, so that will definitely have to come out.

Having also just tilted one seat back to expose the zip, I'm not 100% sure what it does, it doesn't seem to be under any significant tension so if you undo it and leave it open will it affect the seat in any way, other than stopping debris falling through?

I was confused by the zip too, its not under tension.
I like your foam pad solution from the other thread. Much more reliable than Audi's system :)