Specifically related to the last sentence, do you think that adding a prefix at the beginning of some data will hamper the human usability? I'm starting to think that might be the case, assuming people autofill such data from other sheets.
No, but I think it's an unnecessary addition to the format that attempts to fix a problem that shouldn't be/can't be solved in the first place (i.e. making it friendly for humans to directly work with the raw CSV or manually add valid entries in Excel).
The focus should remain on this being a bare-bones software-to-software interchange format.
That it is reasonably not too terrible for a human to look up a single one-off value is a nice bonus. But I think any human requirement beyond that is impossible to satisfy.