This is currently done ad hoc, with each call site that needs to do this conceptually reimplementing the following logic:

Group together as many of these writes as you can into one applyOps entry, and if we would otherwise go over the BSONObj max size limit, start inserting into a new applyOps entry, repeat until done.

E.g, this is currently done in batched deletes and will be implemented in SERVER-117512.

It would be nice to have some standard way of doing this