Just to say it out loud, if that grayed check were used for anything other than indicating a "partially checked" situation, I think that would be confusing and a mistake.

If they has a "grayed unchecked" option, it could be used to indicate "checked" or "unchecked" but not subject to user control, basically indicating that some other condition caused it to be checked or unchecked. But, that's tilting at windmills (unless we build a custom UC with our own images).