Propeller can’t really do many instruction level modifications due to how it works (constructs a layout file that then gets passed to the linker).
BOLT could do this, but does not as far as I’m aware.
Most of vectorization like this is also probably better done in a compiler middle end. At least in LLVM, the loop vectorizer and especially the SLP Vectorizer do a decent job of picking up most of the gains.
You might be able to pick up some gains by doing it post-link at the MC level, but writing an IR level SLP Vectorizer is already quite difficult.
Post link optimization (PLO) tools have been around for quite a while. In particular, Meta’s BOLT (fully upstream in LLVM) and Google’s Propeller (somewhat upstream in LLVM, but fully open source) have been around for 5+ years at this point.
It doesn’t seem like Intel’s BOT delivers more performance gains, and it is closed source.
It was open source, but has since been deprecated.