Ok, my bad, I had the sequence wrong.
On the reading side
using BSON, Sockets
servername = "\\\\.\\pipe\\testiobson"
server = listen(servername)
sock = accept(server)
var = BSON.load(sock)["data"]
On the writing side
using BSON, Sockets
var = <somevar>
servername = "\\\\.\\pipe\\testiobson"
sock = connect(servername)
BSON.bson(sock, Dict("data" => var))