I'm not sure how they are too opaque for that. If you are converting into an intermediary format, and saving that, depending on the format, it may even be easy. Here's someone that rolled their own solution, and they say they can edit it through a regular text editor[1]. Here's a company that provides a solution, which also purports to offer team collaboration[2]. Here's a free set of tools that provide versioning, change tracking,and diffing[3]. That's from a single google search, the first three results. I'm dubious that there's not a way to deal with this problem that fits most the sensible criteria you could come up with, given that this is obviously an need people have been working to serve for quite some while now.

1: http://stackoverflow.com/questions/131605/best-way-to-do-ver...

2: https://www.pathio.com/

3: https://xltools.net/excel-version-control/