Re-lock and unlock bootloader on the Nexus 5X for Android Pay
Currently, No.

Locking the bootloader actually requires a wipe as well now so you would have to back up your device and then restore after locking.

I was in the same scenario with my Nexus 6P and as I didn't need the bootloader unlocked any more i have locked it. I used a combination of adb backup, copying files manually off my device and the built in backup/restore functionality.