TLDR: I couldn't find this info when I was looking so if you want non-Audi centercaps for your CH 028 wheels, use 7L6601149B Touareg centercaps and heat gun the clip side for ~15 seconds before installing. They will fit right in. BBS 09.24.258 caps are pretty close but too small, but can be "made" to fit if you are committed.
When I was deciding on a set of summer wheels for my 2016 GTI, I settled on the BBS CH 028 from the 2006 S4 because they look good, they're cheap used, and have the correct 57.1mm CB and ball seats to use all OE hardware for mounting.
Had them powdercoated and they went right on my Mk7 without issue but the one big issue I had with these wheels was the stupid center cap. I noticed when I was getting info on these wheels that lots of people had trouble finding caps that fit well aside from the Audi ones; some people modified the Audi ones or used silicone or tape to make other centercaps work, but I wanted a better solution.
I measured the face of the wheel center cap indent at about 71mm and the smallest part of the centerbore at about 57mm, and went looking for caps.
Really wanted to run actual BBS caps if I could, so I ordered a single 09.24.258 cap which supposedly measured 70mm outside and 56mm inside to try out. Unfortunately the 56mm inner was way too small to clip in. I experimented with a heat gun and pliers to spread the clips and get it to hold, and was ultimately kinda successful, but it was a pain to do and the BBS clips are $22 each so I wanted to check my other options first.
I read that a lot of people with Audi wheels on VWs were running Touareg caps, 7L6601149B, so I ordered a knockoff set for $15 on eBay. They seemed like they would fit perfectly, but as soon as I tried to push one in the clips snapped off. They are just BARELY too large to fit into the CH 028. So I experimented with the other three...I tried removing tabs on one and bending the tabs by hand on another, neither of which worked, so I brought out the heat gun and tried softening the clips. It fit right in perfectly!
So I ordered another set and carefully heat gunned the back of all 4 caps for about 15 seconds, pausing occasionally to make sure the plastic was becoming more pliable but not getting too hot. They all went right in like they were made to be there and likely won't be going anywhere. I might put more effort into making the BBS caps work down the line, but at least now I have VW caps and can share the solution.