Have you looked at the FolderIn/FolderOut plugins? Those were designed with CVS in mind. Sigil's own checkpointing feature also creates local git repositories for epubs in the Sigil Preference directory. There's no reason those repos couldn't have github remotes defined so that commits could then be pushed. I can't speak to the SCSS aspect, since that's not valid in epubs, but there's a few ways that Sigil can be used to edit content maintained in CVS.

You can use git. Simply use FolderIn and FolderOut plugins to write/load files to/from the repo. We do this with our User Guides for Sigil and PageEdit.

So the workflow is to use FolderIn to load the epub from a github repo. Make any changes you need in Sigil using its editors or OpenWith and your favourite editors and when ready use FolderOut to write the files to the repo and do you commits if you desire.

In addition, Sigil's Checkpoint feature already creates a github repo to store files in in case reversions are needed. That internal github repo is stored in the Sigil Preferences directory. It can be accessed and manipulated like any git repo. Making a Checkpoint creates a tagged commit to this internal github repo.

You could also do something with git subtrees. Have some sort of automated process that generates the css from your SASS and use Sigil to edit the subtree epub source. Just keep in mind that anything done to the css in the epub is not going to be able to be replicated back to your SASS. Editing SASS in Sigil isn't going to be feasible. You're going to need to somehow externally preprocess your scss before using Sigil to preview/edit the epub. As I said: some judicious use of subtrees and some preprocessing scripts could work with Sigil and its CVS options.

Great information. Thanks!