Well, I sure wouldn't want to be a German patriot after World War 2, would you? Hey, it's pretty bad being an American patriot after the Iraqi war, and that's far more defensible.
Yes, irreparable harm can be done to the RP or reputation of a faction.
I am pretty sure this isn't the end of Faction War, so either defeat or victory, while being significant, is not so grave.
Both victory and defeat in this particular battle can, in fact, be used to inspire Caldari to fight. For example I could name a couple of operations that will have much greater impact on faction war than fight for Luminaire.
Either way, if you stand in fight, died you or survived, won or lost, you (as a character) did the right thing and have nothing to be ashamed of.
And if you can't be physically in fight, you can just RP that you was inside dat titan, manning one of guns or fighters (though i am not sure it has fighters, but anyway), you got the idea.
Staying loyal to your faction, giving support to the last, no matter how things unfold, and then you won't 'lose' even if you faction 'lose'. So, just, show your firm resolve and devotion of your character.
As of being on losing side, I remember one particular battle to try to defend a system against huge fleet. CTA was called a couple of days before the battle, but only a couple of peoples showed up. I think we took major plex, i was in a battlecruiser, and then they arrived and went in. Of course we just died, but luckily we did manage to kill something. System was lost, but we gave fight without slight chance of victory, and even score a kill. And it was glorious.