No, there is no precedence (apart from the linked draft) - but why not?
The intention is to have a question Sandbox for questions on Main. For example I remember that there were arguments against it being used for questions+self-answer, but it was used like that once (10k link) after a little meta discussion.
Personally I don't see a lot of harm in using the Sandbox for Proposed Questions for question drafts that go to main and question drafts that go to meta.
Especially because the traffic on meta is so slow and meta is much, much more lenient about on-topicness I suspect there will likely not be a whole lot of cases where people will want to use the Sandbox for drafting their meta question/discussion. But if you feel that you could use some help with grammar or collecting a few previous discussions about a topic or something like that the Sandbox is probably a good place. Just be explicit about it being a question intended for meta, link to meta-tags like
[meta-tag:discussion] and add a Sandbox note about what exactly you expect from drafting it in the Sandbox instead of simply posting it directly. Also, you may need to expect less feedback. The main goal of the Sandbox is to help new users that have problems with main site questions. Meta discussions are really not such a big problem as far as I can tell, so people trying to help others in the Sandbox may very well focus their attention on other drafts if there is more to do.