This is interesting, but I'm not sure it actually makes much difference for realistic workloads. ffmpeg probably has the most link-time dynamic linking of any Linux package, and ffmpeg -loglevel quiet takes only ~40 ms on Alpine and ~60 ms on Debian. Other programs using ffmpeg tend to either statically link it (Chromium) or link it at run-time (Firefox, most video editors), neither of which would be improved by this optimization.

Would it be nice to shave 60 ms off of every ffmpeg/mpv invocation? In isolation, sure, but considering the maintenance burden and potential inconsistencies I don't think it's worth it. Nix is supposed to ensure that the dependencies are always the same, but currently if something breaks somehow, the wrong version will be loaded or an error will be emitted, whereas with this optimization, it will crash or silently invoke the wrong functions which seems extremely difficult to debug.