Building on SERVER-16565, please consider displaying a warning message when db.fsyncLock() is run when WiredTiger is running, stating that fsyncLock is not supported for backups with WiredTiger.

The messaging would be along the lines of the text we added to the product documentation in DOCS-4751, namely:

With WiredTiger, the db.fsyncLock() command cannot guarantee that the data files do not change. As a result, do not use this method to ensure consistency for the purposes of creating backups.