Is it possible to add new archive 7z compression formats (different compressing algorithms in one 7z)? Recently, the open source version of PackJPG was published and I would try to integrate a new compression system.
Quick CAVEAT, version 2.5i (2014) can identify PJGs created with version 2.3 (2009), but it cannot uncompress these old PJGs, for that you'd need PackJPG.exe 2.3c (Win32 example, same idea for all platforms.)
I think 7-zip already uses special tricks (pre-processing filters) for PE "exe", "dll", etc. binaries. Maybe it could do something in this direction based on the LGPL PackJPG ideas also for "jpg". But above all any unzip tool (not only the next version of 7-zip) must be able to uncompress the result.
maybe u have more fun with another imageformat
try webp instead of jpg