Let's say your unit with Rage Spine is down to one tray and an enemy suffers the effects of the morale card that has them attack themselves. Will Rage Spine work? You can't actually suffer the effects of the card. In other FF games, the template "Do X to do Y" clearly indicates the cost of X must be paid in order for Y to proceed, but there is no statement like that in the rules, as far as I'm aware. Still, I'm inclined to think you can't Rage Spine if your unit can't suffer the effects.
(By the way, this problem applies to other morale cards, too, like if you're engaged with several units and are missing trays such that you can't reform and you can't march be cause you are engaged.