In the spec it says:

Examples of sorts disallowed by the grammar:
{“x.y”: {$meta: “text”}}

Examples invalid queries with a $meta sort (where a corresponding $meta projection is not correctly specified):
db.collection.find({$text: {$search: “hello”}, {t: {$meta: “text”}}).sort({s: {$meta: “text”}})

Yet this appears to work (and sort using the text metadata):

> db.test.find(
    { $text: {$search: "tyler" }},
    {s: {$meta: "text"}}).sort({"WRONG.\^$.$$$": {$meta: "text"}}
)
{ "words" : "tyler brock", "cool" : true, "s" : 0.75 }
{ "words" : "tyler brock awesome cool", "s" : 0.625 }
{ "words" : "tyler brock cool cool", "s" : 0.625 }