TFT systems did not have morale, and when playing D&D, we never rolled against morale anyway, we just used it as a guideline.
I toyed with potentially adding morale to HOW, but it seemed like a needless stat addition.
That being said, in the Silver Dragon adventure from Metagaming, there was a morale system inserted:
XI. MORALE DIE ROLL"Each time an opponent in an enemy group is downed make a die roll for each remaining opponent. Each opponent will react according to the one die roll made for him. This reaction may be for the combat round to follow the current initiative roll or for another time period designated. Add one to the roll for each opponent previously downed.
"Die Roll Reaction 1 - 3 Continues fighting, no reaction 4 Hesitates, will not engage voluntarily this turn, but otherwise fights normally. 5 Hesitates, will not take any attack option this turn. 6 - 7 Break and run, will try to disengage and/or run off map away from danger. 8+ Surrender - opponent drops to knees, drops weapons and tries to surrender."An opponent who rolls a Break and Run will only Hesitate (no attack) for one turn if he is the only one to have the Break and Run reaction. An opponent who is Engaged when he rolls a Break and Run will try to Disengage for two turns, if unsuccessful he will then continue to fight normally. An opponent who isn't engaged will run from the map at every opportunity at top speed, dropping all weapons and trying to avoid the adventuring group. Any opponent or Character who leaves the combat map is considered out of the battle and can't return."
I think a "fight or flight" check is an interesting option. I have in mind it would come when an creature (or character?) is at 1/2 ST and again at 1/4 ST with a +1 modifier to the d6 roll at this point. The roll would be done using a version of the metagaming chart above.
A modifier would apply to the roll based on how the overall battle was going. So for the testing creature, if your side outnumbered your foes (-2 to your FoF die roll) same number as foes (0-no modifier) If your side is outnumbered by your foes (+2 to your FoF die roll)
Still pondering some different options (and this would be an optional rule) but I thought I'd share this work in progress all the same.