I realize your question is a little old, but I’ll post this for the sake of future googlers.
I export all my NFS shares using the “Advanced Edit” button, like so:
/export/export_name *(rw,async,insecure,all_squash,anonuid=1000,anongid=1000,no_subtree_check)
all_squash: maps all connections to the anonymous user
anonuid: change the anonymous user from nfsnobody to a user you created that has rights to the btrfs share. Get the uid from System -> Users.
anongid: same as above, but the group ID.
no_subtree_check: Nothing to do with permissions, but disables subtree checking, which can cause problems when a requested file is renamed while the client has the file open
This might not be what you want to do from a security perspective, but for me this is just for home use so I’m fine with it.