What do you mean by you want fschange to "diff" the files? Do you mean you want it recorded whenever the contents change, or do you mean you want the contents of every file version recorded? (Basically, do you want fullEvent=false, or fullEvent=true on fschange?)
If it's the latter, I don't think you need to do anything. When you want to diff the file content, you can simply use wildcards in the source name when you query for it, rather than a specific file name. If you want, you could create a TRANSFORM to modify the source name into a "base" source that doesn't contain a timestamp, so you'd search on
source=config rather than
If it's the latter and you don't want unchanged versions recorded, I think you could use monitor instead of fschange, and in props.conf set CHECK_METHOD = entire_md5 for the file.