It's hard to find any documentation on DBRefs.

But I believe the intention is that the $id field can be anything that can be stored as _id.

The JSON parser however insists that it is a quoted 24 char hex string, or "ObjectId(...)".

See:
https://github.com/mongodb/mongo/blob/master/src/mongo/db/json.cpp#L663
https://github.com/mongodb/mongo/blob/master/src/mongo/db/json.cpp#L467

These should be valid objects:

fromjson("{myref:DBRef('coll',1)}")
fromjson("{myref:{$ref:'coll',$id:1}}")

But they both fail.