kinda going a little off topic, but it would be kinda cool if you could make arc effects avalible as a glowpoint option so you could have a localized arking effect that is always visible, you could use aurora to add them and it would be done like a localized or vectored glow. arc points arc around the point unless there are several points in close proximity, in which case they will arc over the gap. radius could be used to specify a field of influence. if two arcpoint's field of influence intersects with another then the arc crosses the gap (even if the arcpoints are in two different models). think of that effect on the sathanas fleet as they powered up to destroy capella. alternatively you could add them to asteroids to have a localized electrical storm.
for the original idea, i dont think using a bitmap would work. every time i use a bitmap for a thin beam it looks kinda choppy. instead could you not generate some sort of aura around the line and do some aa. it would be faster and look beter i think. texturing everything isnt always the answer. it is possible to enhance the arc effect without using a bitmap.