I bought a DAC and using it successfully. While using I had to modify my pi case and the top cover and one side cover had to be cut with the RCA jacks protruding above the case height. I was just looking at the design. It seems to me that if provision for on-board RCA jacks are removed and a single pin stereo socket (slim-type as you are already supplying) is fitted off-center (marked in red in uploaded pic) one can fit the same comfortably in the same case with just making a single small hole in one side and the case maintaining its shape. In that case people wanting to use RCA may utilise the already supported RCA soldering points provided on-board and arrange to fix RCA jack anywhere as they wish.
Just my thinking.
Suggesting PCB Design Mod