Option : javap -l -p -c -constants %s > %t 2>&1 : doesn't work for me on beyondCompare4 4.3.7 (latest version to this date), and if used directly through cmd, gives a .class output but the formatting is bad and it is not really interpretable for the java reader I am.
Option to download the format through http://www.scootersoftware.com/download.php?zz=kb_moreformatsv4 : doesn't work for me neither. This library is based http://varaneckas.com/jad which is quite old.
The option I recommend and that worked is :
Download https://github.com/kwart/jd-cli (project based from the famous "Java Decompiler" or "jd-gui" as we know it). Once it is installed, do the following in BeyondCompare4 :
Go in format, create a new format, use external program, and use the following :
C:\Users%USERNAME%\jd-cli-1.2.0-dist\jd-cli.bat %s > %t 2>&1
Hope this will help some.