Vicott (creator of UI Tweaker and other great free apps) has released a portable PPC version of UPX to compress your files right on 6700!
It can create a backup as well as uncompress. Attached is the zip.
You can read his extended
post on the subject detailing how UPX works and what it does for your PPC.
I personally swear by this app as it can great shrink down programs like Skype and Opera saving MBs of space on your device. (though it is
not recomended to compress skype and opera on the ppc as they may be too big, use the desktop version for those)
Give the program a shot...
Read his blog for other stuff here.