How about every time energy is poured into a hull (not blast energy, but coherent energy - lasers and beams), it increases the self-illumination level at that spot a bit. The exact amount would vary from weapon to weapon, as a factor of their damage-per-hit level.
It would gradually wear off (cool down) at a constant rate (X amount of self-illum levels per sec, so that glows from high-damage beams would take longer to cool down than laser hits), but additional shots to that spot would, of course, be additive. And lastly, the size of the glowing spot itself would increase depending on the amount of energy being absorbed. Say an 85:15 ratio of self-illumination intensity to self-illumination size.
