Ok so gave it some thoughts, and realized... "
why not just use ztool for scan directly"?
Because, to me it seems GFS and ztool are about same speed and only difference is disk writing process on the ztool side. And ztool not only
never hang, it give you 100% accurate result of what you would actually get during recompression!
So, this calls for cmd scripting, I only tried basic one if it work on single file. And it does:
calc.png
Just download dd for windows and pipe it to ztool. Make sure you /dev/null output.
Now, to make it more complex batch script for whole directory.... I guess tarring+pipelining would do(or 7z -m0 "store" + pipe), but for now you get an idea. It doesnt give you all the info, but then again what more you need than seeing size differences + this is 100% accurate to real result - no false positives and no missed streams.