Google has a quite excellent jpeg compressor (slow, but creates very small jpegs)
It would be great, if there was some functionality, which would use that compressor to compress the resized, cached jpeg-files used for output with guetzli.
The best idea I have, is writing a script that directly operats on the cache directory outside of neos.
Would there be some more native way how to do that?
However you could easily fork that package and adjust it to guetzli or feel free to submit a pull request to that package to add support for using guetzli for JPG optimization.