Light Flow not pushing email notifications to my Pebble
The issue is actually caused by the feature that I wanted to make use of: sleep mode.  For some reason, if the option to disable Pebble notifications while in sleep mode, all Pebble notifications are disabled, regardless of sleep state.  (The release notes indicate that this was already fixed, but that apparently isn't the case.)  Since I can disable notifications at the watch (thanks, @GAThrawn), I have no need for preventing Light Flow from sending notifications to my Pebble while I sleep.

Simply disable the sleep mode check for send to Pebble in Light Flow, and notifications will start flowing.