This is a good start. I have been trying to get some convincing lightening too. Initially I too tried to do a night scene so I could get the cloud illumination effect, potentially very impressive indeed! The main issue I ran into was that the light source would naturally extend along the length of the bolt and not be limited to the extremities. Ideally a well modeled bolt with its luminance value set to something suitably large may do the trick (I would guess up in the millions!). Lightening a scene only from a luminance source theoretically should work but I found the various quality levels had to be set too high to get anything respectable in a reasonable amount of time. (ie lots of noise !!)
I think your bolt needs to considerably thinner, not necessarily real world size (a few cm only ) but maybe more like a meter in width and quite a lot more complex, more tendrils etc. I don't think we're going to get any kind of "glow" feature in terragen anytime soon so that will be a post process job.
I look forward to seeing this progress.
Enjoy your camping, going anywhere nice?
Richard