I've noticed many mods with heat shield additions (e.g. MOLE, DSEV) give the heatshield ModuleLiftingSurface as well as the stock heat shield module. Would it be possible to give heat shields a small amount of lift? (if not already implemented)

Yes and no.
You can add a static amount of lift through the part config. I do not think that I can manipulate it at runtime though so that it is proper for the rescaled parts/diamter adjustments.