[Rhys]

I remember a while back a site showing off a custom SNES with a bunch of extra features, I believe it was done by the same person that did a fairly extensive German translation of Breath of Fire 2 if my memory serves me correctly. Does anyone remember this / know what this mod was called?

[Jorpho]

That would be d4s.  Much Googling suggests you are thinking of the ULTRA16.  The archive.org copy does not seem to be very complete.
http://ultra16.dforce3000.de/index_ger.htm
http://snesdev.antihero.org/?cat=7

I distinctly recall when he was doing the BoF2 translation, there was a picture on his blog of his dev unit – which featured a BSX with wires coming out of the memory pak slot.  I can't seem to find that right now either.   Here we go:
https://web.archive.org/web/20060208020400/http://bof2.blogspot.com/

[Rhys]

Yeah that's it! I was wondering what happened to it, I know he sold a few of them but I wasn't sure if it was still a thing.