I understand about using a different texture on the model, but that would require a whole new render. What I mean is taking the pre-existing 2D renders used for stock footage, then adding a dirt/grime/whatever layer over *that*. Or would that be more trouble than just re-rendering the scene using new textures?
It's possible, if the difference is purely in the texture, but it won't necessarily look that great. For one thing, you wouldn't be able to do anything fancy, like have modeled or bumpy damage. Actually, even having burned areas not be shiny like clean metal might be difficult.
Now, I don't have any production experience, so I'm essentially talking out of my ass, but I'm not sure if that'd really be necessary from the savings perspective. The hard part would be making the damaged model of the ship in the first place, but once you had it, new stock footage shouldn't be too difficult. If you so desired, you could swap it into the old 3D scenes of the stock footage and re-render it, and I'd imagine fresh renderings of old scenes would be the least-expensive VFX you could buy.. In fact, this exact thing happened with some stock-shots in nuBSG, when a damaged version of the ship was created for season 3, and an even more damaged version for season 4.
Just adding a grime layer to the composite would be less trouble than an all-new composite, but not much less.
I'm suddenly remember the DVD commentary for "Police Squad!" where the producers commented that by having the actors just stand still at the end of the episode rather than doing an actual freeze-frame, it saved them upwards of $12 on the optical effects.