As is, any changes to CSS via the Upfront editor

As is, any changes to CSS via the Upfront editor are minified and inserted into whatever template file (single.php/etc) at the parent theme level.

With future releases, would it be possible to–either by default or via setting–have CSS changes save to an override template within the child theme (Luke & Sara, etc).

I was able to perform the latest Upfront upgrade without any issues/lost data, but I foresee this as being a potential issue as your dev team moves forward with the plans to make the Template files more accessible (and therefore pushing changes to those template files in the core updates)