My 'system.dat' is about 12M, and I just tried the above. Unfortunately, the "regedit" step stopped at 74% complete (no progress for 30 minutes) and I was forced to reboot. In DOS mode, I observed that 'system.dat' was now about 9M. I then did a plain vanilla 'scanreg', which reported the registry corrupted and restored a backup copy. So it looks like the above procedure cannot handle a "large" registry either.
But you can also export the whole registry to, say, RegFile.reg, then go to pure DOS (not a DOS box) and run:
regedit /c <path>\RegFile.reg
You'll get essentially the same results.
NB: The above applies to the registry *compactors*.
BTW, while I was able to find the Japanese 'regcon' package (version 3.11, file 'rgcon311.lzh') via the Internet Archive, the links to the English translations here are defunct, so I was unable to try this.