I'd been pondering this a little while. Famously, the CC0-1.0 licence is not approved by the OSI. It's not disapproved either—it exists in a weird uncomfortable state where the OSI says that "yeah if this goes through the approval process we might not approve it", and they mutually agreed that not having it on OSI's list would be better than the OSI explicitly disapproving the licence. Something to do with patents or moral rights.

That's my basic understanding, anyway. I should've looked up the details before posting this.

@Flameeyes mentioned in his blog post that Google won't allow usage of stuff licensed under CC0-1.0, preferring Unlicense. According to another post, Fedora has the opposite recommendation—preferring CC0-1.0 over Unlicense.

So none of that's great.

I think there may be a super simple workaround, though. Instead of exclusively licensing "uncopyrightable" stuff under CC0-1.0, you could dual-license it, so that the recipient can choose which licence they prefer.

That looks like SPDX-License-Identifier: CC0-1.0 OR primary-license-of-project. This way, if a user would love to use the software were it not for CC0-1.0, then there's no problem. They can just pretend that the file is under the primary licence instead.

One problem with this recommendation is that it's not super straightforward. "Just slap CC0-1.0 on uncopyrightable files" is a much simpler recommendation than "slap CC0-1.0 OR your-primary-license on uncopyrightable files, in case some people don't like CC0-1.0".

Of course, you could also forgo CC0-1.0 entirely, but that wouldn't translate intent very well.