This could be enabled by taking a backup cursor during lock and releasing it during unlock.
MongoDB changes for WiredTiger 2.7.0
Display warning when running db.fsyncLock() when running with WiredTiger
Backup/restore test - enable fsyncLock for wiredTiger
File system snapshots for backups under WiredTiger
fsyncLock now supported with WiredTiger
Fix backup_restore.js test performing rsync test on mmapv1 storage engine